Theoretically, if someone tracks all commit hashes for a repo on @GitHub, they can view the repo's past, present, and future history, regardless if it has been replaced or deleted.
This is basically a git multiverse. ✨
I have asked @GitHubHelp (ticket 1466521) if it's possible to clear these dangling references.
Sadly there's no way to remove those unless all forks are removed, which it will never happen anytime soon.
I guess there's a reason why we don't mess with the multiverse.