There are open issues for Kbin, Lemmy, Mastodon, MissKey, Firefish, and Pixelfed about OIDC. Some projects have implemented limited OpenID/OAuth2 services for logging in with Google/Facebook/Apple, but for most services this really depends on someone getting their hands dirty and implementing the OIDC properly.
All projects seem to have much bigger fish to fry in the mean time. I don’t think we’ll see this happen without an external (team of) volunteer(s) taking up the tasks and implementing the feature in some kind of unified way.
I don’t think this should be particularly hard for most services, except maybe Lemmy, because many projects already support external authentication. This just needs some implementation, testing, and perhaps a security review to make sure you cant authenticate yourself into other people’s accounts.