Open source chat software:

IRC - needs a ton of extensions to even try to work like a modern chat, no federation, idk didn't use it much

Mumble - no federation, need your own server to have a private talk since no encryption

ActivityPub/Fediverse DMs - No encryption, unreliable privacy, do not use for actual private conversations

XMPP - loses encrypted messages, there is like, one good client I saw

Matrix - doesn't have a good clients, loses encryption keys and generally very confusing in that regard

Telegram - not open source, shady, no encryption by default

Signal - Encrypted, pretty good, but not decentralised and requires phone number

Session - Bad UI/UX, unreliable, slow, buggy, fails at showing messages in proper order

SimpleX Chat - IDK what it is, seems to be better than Session with simmilar idea, but no audits and still bad UI