In an ideal (and therefore unrealistic) world, it would be done by the UX designer/dev equivalent counterparts of the ones developing these open source softwares.
Why do low level engineers work freely on FOSS but designers or UX devs don't is the question.
Proper UX designers would be ridiculed in the FOSS world because everything is bloat or too corporate. If it doesn't look like Windows XP or a 70s terminal it isn't welcome in FOSS.
Gnome is probably the best example of good design in the FOSS space and look how people talk about it. Even in this thread you have people claiming being able to view images and video inline in a chat is an "anti feature". Can you imagine trying to design a product for people so detached from reality?
Because they have different opinions doesn't mean they are detached from reality. IRCCloud shows images in the feed, for instance. I just like the fact that I don't need that overhead in my client if I don't want it.
If open source/community, then chronically starved for resources and contributors have divided directions.
If commercial, then they want to differentiate and do embrace, extend, extinguish, in order to drive everybody to their client.
I have come to believe, that the protocol spec and servers, is just the easy part of chat apps.