Is that true? I think all the apps that I had to partially enable notifications I managed to do so. To the point that I started to wonder if Google is requiring it as part of the app review process
They are used but they definitely don't enforce strict separation (which of course doesn't scale, but they could at least penalize apps that get reported by users for violating it). In most apps there is one notification channel that has both the important stuff you want and some marketing garbage.