Turns out I've been pruning my #BorgBackup archives wrong. There's a second step involved that actually deletes the "removed" data rather than just (presumably) removing the refrences to that data from the index. While I knew about this second step, I assumed that it was like how it works when you delete a file: the data technically remains intact, but gets overwritten by subsequent writes. Apparently I was wrong about this. I've been pruning—but not compacting—my daily backups monthly for years.

Long story short: I just spent an entire day compacting my backups. My backup storage just went from 71% full to 38%.