Every notification system eventually becomes email. What that means is that we probably need OS-level spam filters so that you can report things as spam. If the notification looks spammy based on the training, it simply won't show up. Marketers think they're reaching you, an army of software engineers remained employed to figure out how to refactor the API to send notifications with the V3 protocol or whatever unnecessary change the OS vendor made this week (driving up every other software engineer's salary; thank you!), but you don't have to be woken up by the latest not-actually-a-very-good-deal.
Whether or not the notification was delivered or blocked by the filter probably shouldn't be fed back to the application, to stave off the inevitable workarounds. But people will test stuff on their own devices and find workarounds. "Your order has arrived! Tap for details." But when you tap it's just an ad. On the other hand, it would be nice for things like Pagerduty to know that all of their notifications were blocked and that they should escalate to the next person on the rotation.
The "mark as spam" data should be fed back to the app stores, and if the app requested status notification privilege and not marketing notification privileges, their app is pulled until they fix it. (Cannot wait to read the whining about that. "I'm just trying to hustle like some blog post from 30 years ago said I should! ")
In the meantime, I don't really ever turn on app notifications. They usually email me, and what's great about email is I have the Spam folder and the Promotions tab, so I never see most of their communications.