I can't tell if you're being ironic, but Git is already federated. Like, GitHub should be just another remote, and which remote happens to be the source of truth is a policy decision. The sticky part are the value-add, non-Git parts like GitHub actions, the issue tracker, wiki, static hosting, access management etc. But most of those are orthogonal and federating them (perhaps as git repos themselves) seems like a good idea.

(disclaimer: work for MS, not on GitHub, also hi again superkuh)