The big and growing issue is that too much functionality is in GitHub and not in Git itself. So while you can move or mirror your repository very easily, moving your issue tracker or pending pull requests is a lot harder and comes with huge loss of information (e.g. there is no way to contact the submitter of a bug report, as all you get is a GitHub username, not email and GitHub doesn't even offer PMs).

That said, I'd happily ditch GitHub for anything more distributed, e.g. hosting Git repositories on IPFS, integration with git-bug, etc. You can mostly DIY that today, but a hoster that provides some free storage would be very much welcome to help with availability.

Another more basic thing I am missing today is a redirect service for repository names , having https://github.com/User/Project.git spread all over the build files makes it hard to move hosters or provide backup repositories. GNU Guix has mirror:// to solve that, but that's about the only place I can think of with mirroring build in.