Slack collapses threads similarly as Reddit. And further responses in the threads get displayed to (and notify) only the people in the thread.

Discord - every further response gets posted as "quoted noted" in the main channel.

Slack has an opt in checkmark to repost the thread response at 1st level note "to all", but it doesn't spam everyone by default.

I also prefer slack over reddit that when I click to "open the thread", it rolls up the whole thread, not just it's chunks.

On the other hand, on reddit, I prefer the unlimited, and clearly visualized nesting/forting. So that people can dive into independent rabbit holes on invividual pieces of the threads without adding noise to others.