GitHub wikis are just git repos themselves, and I clone mine. It's easier to edit them locally, too.
As for issues, I've suggested to GitHub that they make them available via git, and they said simply "I have passed your suggestion on to the team to consider adding the ability to clone Issues". I don't think they realize how nerve-wracking it is to put any data at all in GitHub issues. Even their recommended backup software is out of date and doesn't back up Issues completely.
This is typical. Most businesses don't duplicate their workflow stuff to a parallel system because database backups are sufficient.
GitHub, even with relatively recent issues and downtime, has better infrastructure and skilled personnel than most companies hosting their own instances of these services on some reclaimed box.
Even if they have great infrastructure, this article is evidence that they can still take you down at any given moment if they so much as feel like it.
I haven't checked GitHub specifically, but most of those terms include, "and we can ditch you any time for any reason, maybe with 30 days notice if you're paying"
Not to mention it's also common to include "we can change the terms at any time" - and of course, there's also the fact that this issue in specific has nothing to do with a ToS violation, but with permissions on the parent repository, by relying on them with no backups you're not only subject to their staff's whim, but to the whim of any bugs or "features" in their code.
If you want to take advantage of their infrastructure, that's fair, I understand, but at the very least, run some tooling to backup issues, pull requests, wiki pages, etc on a regular basis.
As for issues, I've suggested to GitHub that they make them available via git, and they said simply "I have passed your suggestion on to the team to consider adding the ability to clone Issues". I don't think they realize how nerve-wracking it is to put any data at all in GitHub issues. Even their recommended backup software is out of date and doesn't back up Issues completely.