@srtcd424@mas.to @be@floss.social @jonny@neuromatch.social I don't have any reason to think they couldn't migrate off of GitHub.
First, mirror elsewhere. Then, move (reading) clients to the mirror. Then, move (writing) operations & communication over to the new system; GitHub becomes a mirror. Then, deprecate GitHub; make old clients emit a warning if you can (probably not). Finally, turn off GitHub.
Possible, but this takes a _lot_ of time if you want it to be graceful, never breaking clients. In fact, each step will take months: planning, execution, communication, bugfixes & refinements. The whole thing will take years.
Absolute mayhem if you're forced to move or, even worse, restore elsewhere without warning.