Now I want a mattermost version of this; the mattermost native client for linux is so terrible as to be nigh unusable (I need to restart it every 24-72 hours as it just hangs regularly).
You could try this libpurple plugin for Mattermost [0]. Should work fine with finch if you want it to live in a terminal or tmux session or something, or else Pidgin or Adium if you want a more GUI look.
Shameless plug: I'm one of the developers of https://github.com/matterhorn-chat/matterhorn which is a terminal-based Mattermost client for Unix-like systems. I haven't used slack-term so I don't know how it compares feature-per-feature, but we definitely try to make most day-to-day features of Mattermost usable from a terminal setting.
I actually do use this, but it is missing a lot of useful features, some of which seem unimplementable in IRC
One example: the mattermost clients won't register a message as "read" until you switch to that view, so if e.g. I check messages on my phone, I will see unread messages, but IRC doesn't have a way of communicating this between client and server so matterircd seems to mark all messages as "read" with mattermost)
Haven't tried this plugin myself though.
[0]: https://github.com/EionRobb/purple-mattermost