Yes, once a backup is complete (completed or aborted) it is safe to delete these things. Also,
This subspace, specifically, is used for tracking progress of the current active snapshot being written during a backup. It is not used by restore.
The backup configuration for a UID was initially left behind on purpose because it was small and may be useful later if there is no external record of backup actions. Unfortunately, as you’ve discovered, it is no longer small largely due to the snapshot range file map which was added some time later during backup’s development. At the very least, this section of the config should be cleared after a snapshot is completely flushed to the backup destination.
Thanks for finding this, I’ll try to get this into FDB 7.2. It’s a very small fix.