We have an !xmpp@slrpnk.net community here on Lemmy. Our SLRPNK instance also gives an XMPP account to every member automatically and hosts a Movim webclient for easy access.
XMPP is certainly more popular for private groups and 1:1 chatting so you will not find that many large public channels, but there is a search engine here: https://search.jabber.network/tags/
The JoinJabber project also has a curated list of recommended channels and communities: https://joinjabber.org/docs/faqs/rooms/