I am still active in some private irc servers. The communities haven't changed much since the golden era of irc.

IRC's not as popular as in its heyday, and while once it was the main choice for multi-playing gaming chat (Quakenet et al), that's largely gone elsewhere, but it's still very good for certain technical channels.

IRC has also proved to be remarkably resistent to commercialisation, mostly due to the users. Even when one of the biggest networks, Freenode, got taken over by a drug addled mentalist Reference who started insisting all all kinds of strange things, the users just upped sticks and created a new network. A bit of fuss, but the important stuff stayed the same and it's continued much as before as a new network, Librenet.

Discord is closed source and has no way to easily archive/record conversations. This makes it unsuitable for a lot of open source projects who need a chat client. I've not used much Discord but potentially the "gamer" culture might put people off.

Matrix seems good but it's not quite there yet from what I can tell. It's got way more features than IRC but none of them seem to work that well. Like a swiss army knife full of blunt tools.

For IRC I'm on the libera.chat server. Usually hanging out in the gentoo channels since I use that distro. There are a lot of different channels for the various devs, user tech support, niche uses like gaming* and also offtopic chat channels.

*More gamers tend to use other linux distros for some reason

It is. It's obscure enough for the normies to stay away. Which is its main feature

IRC still seems to be pretty active in piracy communities. At least most of the private trackers I'm on host an IRC instance.