What about zulip? They seem to fare well in terms of features against slack and mattermost.
It is very polished, functional and easy to write extensions for! It is slightly annoying that I feel like none of these will last a decade and wonder why we couldn't have stuck with XMPP or an extended standard and figure out federation, privacy and enterprise.
Unfortunately, outside of tech, everyone is moving to Teams!
Alternate perspective: they never left Lync/Communicator - it's just been rebranded.
I do miss GIFs, but eh.
When you chat on a channel (Slack, Mattermost) and people start fragmenting the discussion into several topics that interlace... Ugh! I yet have to meet people that would at last utilize Threads.
Slack threads are an utter abomination. I'd rather they removed the feature entirely; they're harder to keep track of than the linear stream of a channel (which is saying something, because the chronological, linear stream is already pretty bad for ongoing discussions)
And if you're really sure you don't want to use topics, you can always just make a single "general" topic in your stream.