This is well-documented[1] in their API. You can list a repo's PRs and the associated metadata/links that make most of the github ui work (code reviews, linked comments, commit statuses, etc).
Having a documented API is nice, but I still wouldn't call it "trivial" to migrate out of.
Besides, we don't know if this API is going to stay around forever or whether it will be discontinued. (Unlike, e.g., the git repository itself, which will always be self-contained so easy to migrate away if needed.)
[1] https://developer.github.com/v3/pulls/