Can you suggest a pre-Slack groupchat, free-to-use, whose (cross-platform) UX doesn't suck? It seems like a pretty low bar but somehow no-one ever managed it until Slack (Skype was almost there for a while but their UX was never great and it declined over time especially on *nix ; AIM actually got it right but only after everyone had already left). Just basic stuff like: notification when mentioned specifically but not on every message, working history, doing the right thing when logged in on multiple devices.

(Discord has replaced Slack for me and my friends now, but it only came along afterwards).