https://docs.github.com/en/authentication?query=public+key+u...
https://docs.github.com/en/authentication?query=public+key+u...
Keybase (or similar) would ideally be used for this instead, but they chose to go a very weird route for their tool, and are now disappearing completely eventually probably.
This isn't possible with the "oh I synced your public keys from GitHub" pattern. The user might revoke a key later (we all rotate our keys every six months, right?), and remove it from GitHub but it can still be used to access your service.
Maybe this is OK if you sync from GitHub on every access request (or cache for a short period), using it as a poor man's OAuth, but that's not what GP was proposing.
https://gist.github.com/sivel/c68f601137ef9063efd7 - uses AuthorizedKeysCommand to make a remote server the authoritative source of your SSH keys, so (if you wanted to) you could make GitHub your key provider.
https://github.com/ierror/ssh-permit-a38 - 3 years old but that's not necessarily an issue