The somewhat snide answer to this is that… this is what git already is! Git has built in tools for submitting patches via email — a federated service. It’s actually what git was built for.
https://git-send-email.io
Of course, GitHub and stuff can be a pretty nice interface on top of git and provides features on top of git itself… but for small projects like the Linux kernel it’s perfectly acceptable :).
Websites like GitHub are obviously a lot more approachable, though, especially since I feel like a lot of people kind of grew up with the gmail web interface or something similar and aren’t used to dealing with mailing lists for this kind of stuff (myself included, honestly).
But to be honest… what are we really getting out of federation for something like GitHub? Like… it’s basically just single sign on with extra steps? I guess you can build alternate clients with the protocol for PRs and issues and stuff? But git itself is already pretty distributed, so I feel like there’s not much to do there tbh.