Mattermost does it the best imo: "threads" in mattermost are just reply chains that go in the main channel, and you can open them separately too.
Besides Matrix's whole matrix-y-ness (lol been using it 4 years, still have E2EE problems REGULARLY), they also completely, completely fumbled threads. Sadly, Zulip has no ecosystem, and the near universal gut reaction to it's UX is "eww". All around, just a god damn travesty.
And in the Discords I am on it's a disaster too. Every time a beginner uses one it cuts the chance of getting help by a lot. You basically have to get lucky that the first person who responds will solve it, otherwise you're out of luck.
For me they are the reason I use Slack. They allow us to have a meaningful conversation and not overlook anything important and at the same time allow interested people to dive into more detailed discussions. For comparison, I use Teams for another project and this is madness - people actively avoid discussion in order not to force the other team members to spend too much time scrolling the screen, and search is abysmal. (Note I'm using these for work, maybe if it was for other types of communication I might not care.)
How do you know though? Because...
> and at the same time allow interested people to dive into
Discussions often start out one way and then go off to something else. People can't know ahead of time if they should dive in or not. That's one of the problems.
It does keep it neater but also doesn’t include or invite others into it as easily.
I’ve seen another presentation of chat where the latest few messages are shown threaded in-line inder the message. If it’s a message I’ve clicked into before, it remembers and shows me more of the thread in line.
I think slack could do this well but would be happier if mattermost or something had it first :)
If you're looking for what a platform made around threads/topics could be, then probably Zulip is the most mature and useful solution around today.