Unpopular opinion number one: Teams is not that bad. At least it can do video calls properly which is not something I would say of Slack.
Unpopular opinion number two: Pidgin was better and I don't why we all decided as an industry to go to these ridiculously heavy memory hog electron based clients. I use Slack from Pidgin to this day: https://github.com/dylex/slack-libpurple
It's not IRC, but there exists a working libpurple plugin that you just auth as with your normal user account:
https://github.com/dylex/slack-libpurple
I just use it with Pidgin, but there are curses clients too (I'm aware of bitlbee, there are probably others) if you prefer the IRC-style experience.