Since my quiet little instance was getting a number of spam accounts signing up but not confirming, and deleting unconfirmed users rather foolishly spawning thousands of tasks to announce to the fediverse users none of them should know about, I decided to make new users request an invite.
This has been good. Now these spam signups show as "pending" instead of unconfirmed so I can reject without clogging up my sidekiq (still not sure why unconfirmed is different)...