git will prune inaccessible objects after 2 weeks by default: https://git-scm.com/docs/git-gc#Documentation/git-gc.txt-gcp...
Now, when is git-gc run? Of course, it's "[w]hen common porcelain operations that create objects are run".
So, what are common porcelain operation? And which ones create objects? On second thought, never mind. I'll just try not to leave anything inaccessible for more than two weeks.