edit: the Matrix is back now.

I'm hearing that Cypher Stack is working on bringing it back online. posting here for visibility, given that the weekly Monero Research Lab meeting is due in 10 minutes.

you can still join each room via IRC. see the address list here: https://www.getmonero.org/community/hangouts/

Libera.chat's own web client: https://web.libera.chat/

~~a good desktop IRC client: https://hexchat.github.io/index.html .~~ no longer maintained.

a list of recommended IRC clients: https://libera.chat/guides/clients

edit: I vaguely recall that this helped me make Libera.chat work, sorry for not being able to provide better context: https://libera.chat/guides/sasl