I think this is a really interesting question.

I've had a bit of luck on Discord getting to know people better, but only really on smaller and more well-managed/curated niche servers. And even then it's usually related to casual gaming where there are community projects of interactions.

I haven't had that anywhere else, with the exception of a subreddit I moderated years ago, where I'm still in contact with and chat to the other mods (even though most of us have either gone dormant in that community or left reddit altogether).

I don't really use Mastodon or Bluesky, and I don't think they are especially conducive to deeper connections. I've tried using Matrix a few times but I find the UI poor to the point of being nearly unusable. Stoat (a Discord alternative) looks promising but it's still a bit of a work in progress with a very limited userbase.

Where I HAVE had a lot of luck: local hobby communities I've got into. Stuff like casual sports or niche interests.

ETA: I'm not autistic myself, but I have some close family members who are, so this community is near and dear to my heart.