As someone who has ND and diagnosed serious mental illness (bipolar disorder), I don't want to tell my colleagues because of the amount of people who don't actually know what my mental illness is, who think my ND is made up snowflakery, and who believe soap opera stereotypes. Some people think my bipolar disorder means I have a split personality, for example, or that I'll be happy one minute and sad the next, or that I'll fly into rages over nothing. I've had people believe that I shouldn't have kids because of it. And I'm supposed to broadcast this across my company? I can function just fine most of the time and will tell people on a need to know basis.
An email signature would assume there is a one-size-fits-all solution and encourages ND pre-empting (the way we sometimes plan out how conversations should go in advance, which then completely throws us when the other person says or does something that wasn't on our script, for example, or pre-empt a response and write an email specifically assuming the person will react in a specific way) which isn't helpful and sets us up for social difficulties/awkwardness. What can I say that everyone who I email needs to know? And anyway, in a large company, people with even the best of intentions will eventually get fed up of the information overload and just skim past it.
It also doesn't help those people who work internationally with certain countries at all. It's culturally taboo, for example, in China to talk about mental illness and the stigma is that you're defective. Your contacts in China receiving these emails with signatures about all your DSM acronyms will want to deal with someone they perceive to be competent. Parents in China literally take their kids out of schools if there's a rumour of an SEN child attending the school so as not to catch it. That's where they're at. They will not react well to Aspidistra's signature waxing lyrical about how she needs to only take meetings in the light of the moon due to her photosensitivity, can only wear left shoes due to sensitivities, can't wear PPE with labels, and needs a tea break every 2 hours and all site offices must be stocked with raisin milk due to her delicate digestion (and yes I'm sending myself up a bit there but this is why I don't tell people professionally, I'd rather deal with this stuff myself).
And the thing is, people can say "this is optional" but those of us with ND are more likely to take it at its face value when someone in authority encourages them to do something like this, which means that they might feel safe disclosing something and then feel betrayed or lied to when it inevitably goes wrong, which damages trust with the employer.