I think they used to have a rule about this but then their own apps started to violate it, so they loosened restrictions. What I'd really love to see is notification categories so you can choose which to accept.
Because as they said: android has multiple channels, but the app owners quickly realized that if they classified their notifications, the unwanted channels such as "news"/advertising get disabled. So they bundle it into the same channel you don't really want to disable again.
For example a taxi service that gives you discounts etc on the same channel as the notification that the driver is almost there.
I really don't see a solution to this problem, because the consumer almost always wants some notifications, so disabling all from apples side wouldn't really work either (as customers would be negatively impacted).
It'd be great if Apple found a solution, but I can't think of one that has any chance of succeeding. Maybe their new Apple Intelligence feature... We don't know enough about it's capabilities yet, but it could potentially act as a filter, with the potential for false negatives/positives. But that should hopefully be okay for something like notifications