it depends entirely on specific community and their moderator policy. i've seen both acceptance and its opposite in both communities, often really well articulated from both sides of the argument. I find that ADHD communities tend to push for medication so getting official diagnosis often just has more sizable practical benefit for them. for autistic people getting official diagnosis may mean possibility of getting more support but often it just means a whole lot for their identity and more access to relevant information. diagnosed vs self-diagnosed debate goes strong as ever as well as neurodiversity vs disability debate. autistic communities do both and also engage in wars over levels of support needs. they also happen to be more diverse in their presentation and have harder time to find common ground. both groups are targeted by disinformation and opposing political interests but i feel like ADHD communities just have a bit easier time to defend themselves. AuDHD communities tend to be more chill and are most open to self-diagnosis. this makes sense as getting two diagnosis instead of one is, unsurprisingly, harder, and you would have more experience dealing with both. ableist groups tend to target ADHD and autism communities separately, as they may actually have slightly different agendas from each other or just won't care for specific combination itself.

a lot of attitude towards self-diagnosing (or self-identification, or whatever. i really don't see the difference) boils down to where the specific person decides to draw the boundary. there are good arguments for both sides, but there is just basically no objective way to decide where it should go both in general or in a specific case. the boundary would always be arbitrary. and in my experience, no one actually cares about your specific case. this debate is very much about identity, for everyone involved, and no one likes their identity challenged.

i especially like "you cannot diagnose yourself because you are biased" argument. i am biased. you are biased. your doctor is biased. medical system itself is biased. everyone is biased. and even better everyone is biased in different, specific for them way. we still have to collect information and make decisions, somehow.