Ugh. I've been cursed to have fashionable disorders for some reason. The problems that can be caused by people identifying into them are in some ways quite different to the problems of "gender" self-ID, but I think they do have some similarities (TL;DR at foot of post).
This is probably going to sound judgemental as fuck to some of those who haven't experienced people identifying into problems they themselves have experience of, and to those who prefer to always accept this kind of identifying-in and self-diagnosis, but if someone finds something in here offensive, please know that the people I'm referring to are probably not the people you're taking offense on behalf of.
I have current medical diagnoses of:
— ASD: this is not only apparently considered by TikTok teens (or young people using whatever they're on instead of Tumblr now) to be a diagnosis which can be legitimately identified into , but also by most online autistic communities at the moment, for complex reasons.
(I'm not going to go into the debate about the rearrangement of autism into a single category and the possible effects of that, as it's outside the scope of my already extremely long post, except to say this: here I'm mostly talking about the possible impacts of self-IDers and self-dxers on autistic people whose autism presents like mine — i.e. not always easy for the layperson to recognise as being the result of a neurodevelopmental disability — because that's the only situation I have experience of, not because I think there's no effect on other autistic people.)
I do understand that competent assessment can be difficult or sometimes impossible to obtain, be very expensive, involve long waits and intrusive or upsetting processes, or result in a diagnosis that once official could potentially affect your future options, and that if you don't have access to clinicians with the right kind of expertise, your ASD may be missed. But without objective professional opinion I don't think anyone should be outright saying they are autistic.
It's a medical diagnosis of a neurodevelopmental disorder, one which for some people can result in profound disability and intensive lifelong care needs, and saying you have ASD or are autistic should mean that you've been given that diagnosis professionally IMO.
It's especially irritating when those who are not only self-IDing, but also wouldn't want assessment if offered (and may even believe ASD shouldn't be in the category of "things that can be diagnosed by a professional"), start banging on about how it's not a disability, which the public may take to mean that people like me don't need the little help we get.
In social contexts it's not necessary to lay claim to the actual diagnosis (and it seems kinda dishonest in medical, work or official contexts) — you don't tend to see people being not taken seriously, or not welcomed into the autistic community, when they've been honest and said they don't [yet] have a diagnosis but believe/suspect/are sure they're autistic. We're catching up on several generations of undiagnosed autistics going by current diagnostic criteria; there must be millions out there, so it makes sense to accept and welcome undiagnosed people who suspect they are autistic. I was left undiagnosed until my late 20s, because 15 years earlier, when medical attitudes were only just beginning to change, an old-fashioned psychiatrist blocked my proposed assessment because I understood irony.
Unfortunately, since many autistic people recognise these diagnostic difficulties and want to be welcoming and helpful to those without diagnoses, and because many autistic people do find the neurodiversity and demedicalisation paradigms attractive, autistic groups are wide open to all self-IDers and self-diagnosers, particularly online (in IRL groups I see it less). Alongside those with diagnoses and those who simply happen to be currently undiagnosed, there have been increasing numbers of people who don't have diagnoses and TBH don't really seem to fit, who have swamped some online spaces, and it's changed the atmosphere and focus IMO. Some groups will, for example, jump heavily on any social faux pas around this whole area (and other identity areas), which is… not ideal. I don't use online autistic spaces any more — most stopped feeling like familiar places.
And it's not only that there's a lot of self-ID and self-diagnosing (almost two distinct groups, the former more likely to be extremist neurodiversity and demedicalisation advocates who may believe ASD is not a disability); ASD is also a popular one to armchair-diagnose in others. That can be even more irritating, especially when the diagnosee seems indistinguishable from someone just being an arsehole, or is exhibiting behaviour within the normal range, or when it just seems like those two people are not a good combo.
Neither those jumping to autism as an explanation of behaviour they don't like when they're not qualified to diagnose (and much about the situation doesn't seem to fit well anyway), nor those self-IDing without much evidence and demanding special treatment and/or to be treated as an autism spokesperson, are doing much for public perception of autistic people IMO.
I'm not convinced identifying as autistic has gone out of fashion as such, but it's losing ground online to:
— ADHD: some similar self-diagnosis and self-ID issues to ASD, and a similar set of coexisting problems around the genuine large numbers of undiagnosed people who would meet criteria if assessed. I think its popularity as a (sometimes spurious, usually overconfident) self-diagnosis-stated-as-fact has been strongly ramping up over the last year or two, alongside a strong awareness campaign about adult ADHD.
The self-diagnosis of ADHD seems, even more than ASD, to be taking on a flavour of "thing that people sometimes outright say they have despite having no diagnosis, but often without a lot of the struggles that people with the diagnosis contend with, or that appear to only pop up when convenient". There also seems to be a lower threshold of difference/struggling with life stuff for people to start feeling comfortable outright claiming ADHD as fact, compared to ASD.
Again, I have no problem with people saying they suspect or believe they have ADHD, saying they find the tools and techniques useful, or joining communities (assuming they frame it something like "my probable ADHD" or whatever), as this is another one that I was adult-diagnosed with, so I can empathize with that situation.
— Bipolar disorder: this suffers the multiple-whammy of, firstly, popular choice for self-diagnosis (often seemingly dubiously — though self-ID per se happens less, as it's rarely argued to be as much of an "identity" thing as ASD) and occasional exculpatory get-out clause, secondly, insult or slur, thirdly, occasional target for armchair diagnosis, and lastly, a pre-existing huge public misunderstanding of what it actually entails.
Like with ASD and ADHD, these things perpetuate real public confusion about what it might mean for someone (and the people around them) if they have this diagnosis, and they undermine public perception of the diagnosis, how serious or genuine a problem it is, and whether someone who says they have it is likely to genuinely be someone with a serious psychotic/mood disorder. I've never come across anyone with diagnosed bipolar disorder in the UK whose life wasn't very seriously impacted by it, and can't think of any who expected to be able to act how they liked and use their diagnosis as an excuse.
Of course, like ASD and ADHD, there are a lot of people out there who will later come to have a diagnosis of bipolar disorder that fits their symptoms but who aren't yet diagnosed, and a lot who remain misdiagnosed all their lives. Bipolar UK give an average of 9½ years between first presenting to a doctor with bipolar disorder symptoms, and receiving a bipolar disorder diagnosis, with an average of 3½ misdiagnoses, adding that some research suggests over half those in the UK with bipolar disorder are undiagnosed.
So I can see that there will naturally be lots of people out there without a diagnosis who (correctly) suspect or believe they have it, but a lot of the self-diagnosis feels incredibly dodgy and the things some self-diagnosed "bipolar" people say about their "bipolar" seem to be based on misconceptions, inability to objectively see oneself, lack of professional ability to interpret criteria or of experience seeing what bipolar disorder looks like, or occasionally (sorry) attention-seeking or self-serving excuses. By no means all of them, of course; many will have other mental illnesses that are less well-known, or which are more… undesirable, for want of a better word.
If you have bipolar disorder or suspect you do, this means that looking for support online from a community of people dealing with similar psychiatric issues to you is more difficult, as there are so many people stating they have bipolar disorder whose experiences don't really fit, though again, I've not found this to (yet) be the case in real-world support groups.
— Coeliac disease: okay, people don't usually self-diagnose with or self-identify as this by name, or believe themselves to actually have coeliac disease — and there's also the non-coeliac gluten sensitivity thing to consider, which is debated but which I don't want to touch here — but there was also a massive surge in gluten-free diets being promoted for other reasons, and people deciding it might be a good idea to try avoiding gluten to lose weight or for nebulous indeterminate health reasons or because other people were doing it, which is obviously entirely their choice to make, and of course they weren't trying to muscle in on a medical diagnosis. But I think maybe gluten-free becoming fashionable was a bit of a double-edged sword for coeliacs.
Great in that it led to a much wider range of gluten-free foods being available than would be economic to supply for coeliacs alone (though I think the trendiness is definitely on the wane, and GF seems to be having to shuffle up for plant-based).
Not so good in that, for example, for most of those avoiding gluten, it didn't in reality matter if there was a bit of gluten in their food, so all kinds of things (e.g. cakes in a cafe sharing a cake-slice with gluteny cakes) started to get labelled as gluten-free, which weren't, but would sell well; people saw others on "gluten-free" diets who didn't mind cross-contamination, and might well assume that that must be fine for other gluten-free people too; or people heard about those on GF diets sometimes doing things like "cheating" on pudding after insisting on an arduously-prepared cross-contamination-free gluten-free main, and might start to feel avoiding cross-contamination for those requesting gluten-free food was a waste of extra effort.
Basically, I think the words "gluten free" became a bit of a joke, because they got attached to a kind of stereotype of a dietary fashion-follower who enjoys kicking up a fuss and getting attention for their fad health kicks. There were times over the past few years where the topic of a person asking for gluten-free food was the target of endless mocking jokes — a person playing at having dietary restrictions, getting all the "benefits" of feeling special and getting attention [shudder argh] but without going through real deficiency disorders, social exclusion, gastrointestinal distress, or lack of choice on extra restrictions and costs. The jokes weren't aimed at coeliacs, but I still sometimes feel self-conscious even mentioning gluten 🤣
Probably very few of the non-coeliac (and non-NCGS) gluten-avoiders were, or are, anything like attention-seeking pisstakers — GF became such a big market that anyone like that must've been quite a small minority. I'm sure lots were just trying something out, or for some reason feel/felt more comfortable with not eating large amounts of gluten.
— Migraine: this is a slightly petty one. People seem to have been calling their non-migraine headaches migraines to communicate severity forever. But severe headaches aren't necessarily migraines, and migraines aren't necessarily severe (or even painful at all). And not all migraine headaches feel like you'd think migraine headaches would feel.
I don't know that this is particularly harmful, but it is a bit annoying sometimes 
To me, the potential similarities to cross-sex identification seem to be:
- entering spaces that weren't intended for you, thereby changing them so they no longer meet the original group's needs, and making people feel unable to specify
- amplifying inaccurate stereotypes about the group you're identifying into
- using resources set aside for the benefit of those who are affected by issues linked to their membership of that group
- general public confusion about what being a member of that group actually means and what the criteria are
- expanding the group to include people who don't experience certain core features, meaning that discriminating against that feature is no longer seen as discriminating against the group, and meaning that those who are able to succeed at x in part because they lack that feature are inauthentically held up as examples of members of that group succeeding at x
- there could be lots of different reasons someone identifies into a group, and you don't need to be intending to cause harm to the group you're identifying into in order to be causing harm
- probably lots of others I've missed.
(My apologies for the length of this post; when I don't sleep at all and then take stimulants, I get way more verbose than usual and also find it much harder than usual to edit down. I tried, I swear…)