Can we please demand that Github provide mirror APIs to competitors? We're just asking for an extinction-level event. "Oops, our AI deleted the world's open source." Any public source code hosting service should be able to subscribe to public repo changes. It belongs to the authors, not to Microsoft.
The history of tickets and PRs would be a major loss - but a beauty of git is that if at least one dev has the repo checked out then you can easily rehost the code history.
It would be nice to have some sort of widespread standard for doing issue tracking, reviews, and CI in the repo, synced with the repo to all its clones (and fully from version-managed text-files and scripts) rather than in external, centralized, web tools.