The company I used to work at used GitHub just for git hosting. Code reviews, pull requests, CI, were all handled by other tools. There are probably a lot of others in that situation.
No one does this. There is a huge infrastructure overhead of doing what you are proposing. If I were to guess, not even 1% of the companies are separating git & its contextual data (CI, PRs, code reviews, etc).
I'm not proposing this. The last place I worked for did this. There are plenty of third-party tools that do these things with good GitHub integrations.
If they are integrated to GitHub instead of git, they are probably down as well when GitHub is down.
The integrations were mostly there to notify the tools about updates and ensure they have the latest versions of every branch. You need this for things like triggering builds on branch updates, but not for things like uploading a diff to code review.