[Feature request] Implement message threads in Matrix like Slack Threaded messaging

[MurzNN]

Slack introduce Threads in rooms: https://slackhq.com/threaded-messaging-comes-to-slack-417ffba054bd
Will be good to implement same threads feature in Matrix rooms.

[MurzNN]

Seems this is not so hard to implement like in Slack - on "create thread" event we must create new room with title from current message, and in current room provide link to new room with this thread (and message counter).

[turt2live]

This is also being tracked here: element-hq/element-web#2349

[bendlas]

When starting a thread/room, can we allow to reply to multiple messages?

I think, this would be awesomely useful for including top-level posted answers into threads.
Also, it's a kind of nice reification of the DAG, in that the starting message(s) of a thread can point to messages of interest in their history.

Can this be rectified with the notion of rooms?
Should this have a deterministic lazy-creation semantic, so that people responding to the same set of messages land in the same thread?

[dessalines]

This is also implemented in mattermost already:

[ara4n]

We do have label-based threading support in Synapse now, but Riot doesn't expose it yet: matrix-org/matrix-spec-proposals#2326 and #6288 and #6301

[MadLittleMods]

Threading was added via MSC3440.

And for a client, this has ended up as Slack-like threading in Element, https://element.io/blog/introducing-threads-in-beta/