That will be abused like crazy. Every marketing notification will be accompanied by an "order now to get 5% off your order" message so it can be classified as time sensitive.
Potential abuse is not a reason for not doing this feature. Bad actors will be bad in the current system or the new one.
A couple of years ago: Allow all or no notifications, hard to find settings to turn off
Current: Allow all, but easy to manage from home screen, apps that want to offer a choice of types of notification must figure out how, create config UI, make sure system is stable. Users must find, and each app is different and often report options are not working
iOS 15: A general framework that at the code level, each notification can declare. iOS unifies the UI for managing levels per app. Good actors will have lower costs, bad actors can be detected in device predictions.
I only see benefits, and you can still block apps abusing it
It's not a reason for not adding the feature but I'm sure abuse is something they anticipate. Hopefully there's some way to click on a notification and submit it to Apple as a violation of their guidelines and the app gets pulled from the store once a pattern emerges.
App store rules are a joke, a lot of apps should be banned for violating them, but apple just doesn't give a fuck - and there aren't even proper channels to complain about it.
See “Helping People Manage Notifications” section.