Back on topic:
Unlisted threads are posts tapas staff decided to take off the main website. Unlike deleted threads, they’re still accessible via link, user history, or your notifications history.
Typically threads that get marked unlisted are “troll” threads, or threads that go way off topic in that classic tapas forums ‘2 or 3 people battle something out and it gets just a little nasty’ way.
Typically, unlisted threads also get “locked”, so you can’t post new comments.