It's not super new though, people had private forums in the phpbb era as well, which you could only read after login. Or think about IRC, private torrent trackers etc.
I dunno if there were that many successful private forums; locking off access to unregistered users back then was a really good way of ensuring no new users finding the place back then. Very very possible I just wasn't aware of private forums enough though.
Beyond that though, information was still neatly catalogued into small focused threads whereas Discord's biggest issue is that stuff just vanishes in a gigantic chain of messages.
Was a bit late for IRC's heyday but am I right in thinking it wasn't guaranteed that all users would have access to the chat history? That would create an impetus to log the information elsewhere.
Private trackers such as what.cd tended to attract people with strong archivist streaks which I think largely resolved the issue there. Oink could go down and you'd know that a huge chunk of its catalogue would appear on Waffles within a few months.
I ran a private business forum for online marketing. The locked door approach meant it developed a legendary reputation and meant many more people talked about it than they probably would have if it had been open. The mystery effect.
This also happens with some private torrent trackers; the status of having an account means people put more effort into maintaining the place too.
Online marketing feels like a field that would _need_ a private place to talk tbh; anything public would be a beacon for self-promotion efforts by people who are quite good (or at least relentless) at doing so.
