Interesting! I've been starting to consider the alternatives myself, for similar reasons. How would someone else authenticate to file an issue or pull request? Can they log in via OAuth with Google, social media, or GitHub accounts?
A lot of solutions do allow public registration (if this isn't an organizational project--most of mine are, for example), or for example, if you use a solution like Gitea or Gitlab, it supports mirroring a project on Github (if you want the public to contribute, this can be a helpful pipeline in) and you can aggregate issues and pull requests that way.
Gitlab, in particular, does support Single Sign On (via Google and Github, I believe!)
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