GitHub lets you use them as an oauth provider. Issue solved.

Instance fragmentation is annoying in the sense while you can unify log in with oauth you can’t share settings between instances of the same software. Would be cool if oauth could have a generic user_data field to store json of settings maybe…

That's a worthy goal, but the problem isn't so insurmountable that we have to wait for some theoretical new feature to be available and adopted. There are three dominant players out there, one of which has demonstrated a willingness to screw everyone and the "it's not perfect yet" excuse is getting pretty thin.

Switch to Codeberg today and there's a good chance that this federated login will be supported there when/if it's ever available. GitLab could do it too, and moving there will give you a bunch of nice things you don't even get in GitHub let alone Codeberg.

But it's long passed time to move. Microsoft has stolen our code to feed into their slop machine and enshittified the platform. Sticking around because a perfect alternative isn't available only serves to harden the network effect that keeps GitHub dominant.