Matrix already supports Zulip style threading via MSC2326 (https://github.com/matrix-org/matrix-doc/blob/matthew/msc232...). We haven't hooked it up in Element yet though (and ideally would do much better UI).
I may be mistaken, but it doesn't look hard to add it to the UI if the back already supports it. Is the feature planned for a precise date ? Does the Element team lack money ? Can we pay for this feature, instead of paying Slack e.g. 30 000€/year ? Is the lack of developers the problem ?
What it look busy when fully used? Yes, but sometimes when you are working in an Enterprise environment, you need to follow something across multiple rooms or channels and this would save users a lot of clicking back and forth.
while I'm sure there are oodles of feature requests already going, this would be one that would offer a genuine feature that slack does not.