Oh apologies, I missed that it was a general group. I do think that Facebook is especially bad for this kind of thing. You get a bit of it bleeding through to more general forums (I saw some of it on Reddit UKParenting the other day) but it's not as rampant as on FB. It is something to do with their algorithm, it breeds extremes and discord, so you get pockets of people (usually in groups) who are absolutely insistent that something is true whereas the more general perception doesn't reflect this. It usually starts from something reasonable (e.g. the idea that RF is safer than FF) but it spirals and spirals until you get to a really extreme POV which doesn't really follow evidence, like "FF at age 5 is not safe". It's actually very similar to how OCD reassurance-seeking works and I did read a really interesting article about how it's possible that some of these groups online are largely filled with people with unchecked OCD or anxiety disorders and they are feeding off each other and making the anxiety worse and therefore arriving at more extreme conclusions.
The problem is that if you don't know any better, then the amount of people repeating the same thing tends to make it appear as though that POV is common knowledge and/or fact. And if you spend time in the group and sort of passively absorb this stuff as knowledge, it falls under the illusory truth effect where you sort of assimilate it into general knowledge, and you no longer remember where or how you learned about that thing, you just know it's true, the same way as you know how a queue works and what a banana is and who the Prime Minister is etc.
But I do think the FB extremes can be especially difficult because the problem is that because they are extremes, they tend to be positioned against "those other people who get it wrong" and so people are often on a crusade to educate everyone else in the world about this knowledge that they have gained. Parenting groups are especially bad for it for whatever reason - you see the same thing happen on various topics - real examples I've seen aside from car seats have been weaning, safe sleep, sleep training, screen time, vaccinations, home ed, breastfeeding, gentle parenting etc etc etc, and sometimes you'll see this kind of thing where one of the topics (like ERF) bleeds into a completely unrelated seeming or even general group which just then strengthens the effect. It happens partially because you will get prolific posters who are on multiple forums at once and so they either become seen as a bit of an "expert" or you might not realise it's the one same person but you see the same message repeated in different places and it strengthens the effect of it seeming authoritative and correct but it could actually just be the opinion of one person. You can usually tell if someone is giving a considered piece of advice if you ask them where the info comes from. If they have genuine experience/knowledge they can tell you. If they are simply parroting something they have read hundreds of times in a group, they usually recommend the group (this is often well meaning!) and say they have "learned loads" or sometimes they tell you to "do research" or occasionally they will resort to insults.
Not all ERF specialist retailers are hardline on the ERF part, and even most of them who are still sell and will be happy to recommend high back boosters for older children. Most ERF retailers do subscribe to the theory of never using FF for the integrated harness/impact shield type stage, so you won't usually get advice about forward facing harness or shield seats from an ERF retailer, and some of them choose not to spend time giving advice on high back boosters, but most of them will especially if they stock them for sale.
@GKG1 I don't mind :) It's a bit of a hobby for me so I like to share the info where I can although I do worry I will get out of date. I'm not as hands on with different models these days.