Nesting was around pretty early on, but yeah.

I think what all of these platforms are discovering is that we figured out some pretty intuitive patterns early on and are on a constant loop of wanting to do something different before eventually realizing this and reverting.

There's a hybrid wherein you keep references to a branch in a thread but you still display a flat discussion. If you choose to follow the branch you can but otherwise all discussion sits on one level. I like this way more than default nesting, which can be impossible to follow.

Which slack does already.

This is one of slack's weak-points imo. Discord allows you to reply to a particular message when making a post. In Slack you make a "thread" but then click "also post in channel". This can cause a lot of confusion if all you're trying to do is use it as a reply because most people will start replying to the thread, not realizing you were trying to keep the conversation as part of the regular channel flow

Slack buries threads at the surface level. This is helpful in some situations but can lead to a lot of confusion