It's true that big outfits like Github aim to be around for at least the next decade or so, so arguably it's not a big deal for repos to just sit in user accounts. However, other questions make clear that even if the project is abandoned, the copyright remains with the ostensible owner attached to that account. And anyway, in that case the account has to stay up to serve as an access point for the repo.
I wasn't able to find any information on Github, Archive Team, or Archive.org about where repos could be donated under some blanket surrender of rights. Are there any such orgs either within one of these sites or elsewhere to which a repo can be transferred?