I use pidgin daily with the libpurple-slack connector. It isn't super great for large rooms (I often keep Slack open in the background but have no notifications

I use pidgin daily with the libpurple-slack connector. It isn't super great for large rooms (I often keep Slack open in the background but have no notifications enabled), but it is really nice for direct messages, which is what mostly happens on slack in my company. Pidgin integrates nicely into gnome-shell's notifications, allowing me to quickly see messages and reply without losing focus.