You can still find terminal interfaces for a lot of these things, including Slack: https://github.com/jpbruinsslot/slack-term. I don't use Emacs but I imagine you could integrate that somehow if you wanted to.
From what I heard, Emacs Slack client is better than the original one.
Do you see when the last commit to that project was? Slack really did murder IRC and open chat protocols. Allowing an IRC bridge for a limited amount of time was a genius trojan horse.