Building your software usually involves getting dependencies, & those dependencies are, hopefully, in more than one location—which includes a cronjob to a bare repo, or Alice’s fork on another repo that at least has the latest tags. It should be trivial to point to these as mirrors for the cases where any forge/repository, even the ones held by megacorporations, inevitably go down. Even Nixpkgs itself, while not maintaining their own official mirrors, are mirrored by TUNA. Backups are an important strategy, & the source code should also be a part of that.