How about storing issues and PRs in the actual git repository? They should index them for the UI, sure, but the source of truth should be a branch of the git repo just like it is with gh-pages. It should be possible to file a new issue by committing a markdown file to the correct branch and pushing it to Github. Their hub command line tool and their own client could facilitate adding all the correct metadata. This would allow people to work while Github.com is offline and synchronize everything once the service outage is over.
If you have access to a distributed database that is already the best available for manual conflict resolution, why wouldn't you want to use it to store this kind of data? A Github outage is like a partition when you think about it from a distributed db perspective, so treat it like an individual node died and still allow reads/writes to the rest of the cluster that can be merged back once the node comes back online.
Access to the git repository is regulated; issues and PRs aren’t. Anybody can fill an issue/PR on your repo, but only you (and your team) can modify the repo. You’d need to also store all the comments on all issues and PRs, even closed/rejected ones. In some repositories, that’d be huge.
And the amount of actual data needed to store issues is peanuts anyway.
It’s not. Take something like github.com/Homebrew/homebrew-core. There are 15k closed PRs there. There have been 20 new ones today. It’s not rare to have 10-20 comments per PRs; some of them even go over 200. Add CI status, actual PR contents (git patches), comments reactions, edits, labels, milestones, assignees, reviews, projects.
IMHO having a tool to fetch issues locally is a good idea; storing them in the repo is not.
Also as others pointed out, just because the GitHub app is down doesn't always mean the GitHub git server itself is down.