My understanding is that these commits don't get garbage collected ever. After all they still need to support `gh pr checkout` so they need to store them forever as part of the GitHub repo. Sure they won't be included in `git clone` but that's kind of the point.
I mean the PR data. The main advantage of a forge like Github are being web browseable, working great as a backup, and the PRs (and other stuff like artifacts storage and CI). All of these have alternatives and are optional. And the API is comprehensive enough to not feel locked in.