I also use Libera.chat. But, is there a specific channel for discussion of random ideas that you do not otherwise know what channels will be appropriate?

Although, for some stuff, NNTP might be better.

Are general purpose channels still a thing? I used to meet a lot of people (and maybe even date someone) in #<country> or #<city> channels that have gone the way of the dodo from what I can tell.

Some I guess, but I find it hard to "get into" them (not access, but to become a long-time stick-arounder). Much prefer to find niche channels and talk about the niche and eventually stick around because they're just nice people. Then the niche becomes a plus while in reality, I'm still there just because of the people.

Yeah, me a few friends have an IRC channel for the same reason the parent post have a Discord server. Except I don't have to manage anything.