My /var/lib/jenkins filled up, so I went looking to account for the disk usage.

I found that more then 2/3 of my disk space was being used by orphaned jobs and the vast majority of it was matrix builds where the parent job data was removed and the child job data was not. So I [re-]moved all of the orphaned data such that there was no more and lo-and-behold, an hour or two later, more matrix child data was found orphaned and the build date on it falls right outside of the "keep builds for" threshold, clearly indicating that a parent job had just been deleted but the child data had not.