word of mouth or challenges like Cicada 3301 https://en.m.wikipedia.org/wiki/Cicada_3301

The spirit of the cicada challenge is pretty much opposite to the spirit of those forums in question.

Sure, but they recruit in a similar way.

Is this a rumour, or a fact?

You have to pay $2000 (At least in Maza) or have refs from two trusted members. You're most likely get banned by me or Hennor.

I've read that the biggest thing is referrals. Gotta be in the know or 'trusted' by someone else in the scene

Like with anything underground/dark.

If there is a open and clear way to it, it is no longer underground/dark web.

I bet they are a very paranoid circle, though.

Word of mouth and sometime challenges. I remember, there used to be a Greek forum, pwnb0x (which appears to have vanished from all search engines), that allowed members with referrals or by solving puzzles.