Add support for XEP-0201: Best Practices for Message Threads
Description of the new feature
Many chat apps (Slack, Mattermost, Zulip, I think Element/Matrix either has this or is implementing it) provide functionality for replying to a message and keeping the replies in a distinct thread which does not pollute the main chat. This is very useful in chat rooms that have dozens (or even hundreds or more) of people.
I have never seen this feature in an XMPP client, but it seems as though XMPP as a protocol does support it. See XEP-0201.
I would like to see this feature in Gajim.