The code itself is perfectly safe. With the way Git works, everyone that has pulled down the latest code has a copy of the entire history of the codebase. If GitHub decided to delete the public repo, it would be trivial for one of the maintainers to push it to a different host.

What’s more concerning are the issues, pull requests, discussions, etc. But GitHub has a really robust API that could be used to write an export tool. I’m daft, there are already some in existence. This seems like a good starting point: https://github.com/topics/github-issues-export