This is an immensely useful feature for group chats where more than one conversation might be happening in a chat. We've already run into examples of conversation where a feature like this could prevent significant confusion.
+1. Also note that the implementation of threaded conversations in Mattermost is very usable, imho. Slack's implementation seems less usable, to me.
Giant +1 on this. Threaded conversations are super powerful for low/medium volume groups.
Here is an animation I created presenting the flowdock threading feature:

image 漏 xkr47, license: CC BY 4.0
The lack of this feature is making me avoid Keybase. The non-threaded conversations are so difficult to follow.
Mattermost also has similar threading as Flowdock, but without drag-n-drop between threads. Just don't copy from Slack, their threading is inferior, at least when you really want to follow all threads all the time. And then of course there is Zulip which is technically awesome but some people don't like it for that very reason so it's not for everybody.
If I would design something for everybody, I would perhaps go with something like Flowdock/Mattermost, with everything visible by default, but where individuals could toggle the visibility of the replies in individual threads, and perhaps also a setting to allow hiding new threads by default (slack-like). But imho the default should be to show everything to everybody, and more advanced users could customize thread behaviour to their liking.
Like after seeing some thread about uninteresting_subject_52 one could hide the current and future replies to that thread, and later one could toggle the replies back into view or just view the thread in a side panel without seeing the replies in the main chat..
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Giant +1 on this. Threaded conversations are super powerful for low/medium volume groups.