To push back slightly, Discord says they have 19 million weekly active servers, which almost certainly outstrips the number of forums back in the day.

It just feels like there's been a shift away from the super public phpbb forum style, with distinct posts and threads of conversation.

But I'm not sure that's quite the same thing as all the communities going away.

As some examples from my own Discord:

A server for fans of pretty obscure, self published author Victoria Goddard.

A server for fans of Reiner Knizia, the world's most prolific and famous boardgame designer.

A server for fans of the Lancer RPG.

A server for people who play modern (primarily) Japanese card games.