A great alternative to Slack, in the spirit of IRC, is matrix[1]. It has lots of clients, and a great vision. You use an online client called riot[2] which uses the matrix protocol. There are other clients available[3] including desktop clients. There's E2E encryption coming to more of the clients which is inspired from signal protocol (double ratchet part of it specifically), without Forward secrecy so as to maintai…
Matrix is an extremely heavy protocol, not suitable for business use imo. Riot is really obnoxious to use and get set up, and is also extremely resource hungry. I don't think you're very familiar with chat protocols if you think it's even remotely suitable as a replacement for IRC.
Empirically many people seem to use it fine for business and for IRC-style purposes though :)