This is a solution to a non-problem. Hosting taken down git repos is easy an due to gits design all developers already have the source code. The real problem is hosting issues and PRs in such a way. Github has an API and it's possible to script the backup but source code gets backup automatically so when the takedown strikes it's not a big problem.
Shameless plug, but that's exactly the aim of https://github.com/MichaelMure/git-bug . It's a distributed bug-tracker: it stores issues (and one day, PRs) within git. You can work offline and you always have a full copy of everything. It also has bridges for Github, Gitlab and Jira.
git-bug is a pleasure to use so I'm attempting to get it running in the browser using wasm, to create a decentralised github on p2p storage. I'm targeting Safe Network but the same approach could be used on anything with a storage backend, from NextCloud to IPFS, even [cough] AWS.