Session

You can probabaly use element.io or some other matrix chat apps. Those are pretty “anonymus” if you want it to be. Or you can use signal.

Briar, Session, or self host (e.g. XMPP, OnionShare, IRC, etc.)

SimpleX might be worth a look. Although, I think the developer eventually wants to pursue a two-tiered business model. Otherwise, for something like Matrix (where there are several client apps available), no-one’s forcing you to give over your actual details.

What exactly do you mean with anonymous? Group chats with random, unknown people; or tracking free 1:1?

SimpleX Chat and Briar. Also check this link out.

https://github.com/pluja/awesome-privacy#instant-messaging