alt Hacker Newsbibliotek is an interesting one, and one of the most exclusive trackers around.
They have very high requirements as their entire purpose is archival. You can't download anything without uploading new content first.
It's probably the most complete archive of niche books in existence - it's a librarian's wet dream. No wonder LLM training used it, and probably paid a lot of money to someone for access.
A Faustian library, containing every book known but you aren't allowed to read them all without being evicted from the library.