I thought that was already a feature in Nougat? Doze Mode? I guess this will completely force apps to freeze and hibernate, such as Facebook and FB messenger which somehow bypasses the hibernation and force push updates regardless of battery saving measures...
Exactly -- an app that wishes to override the system's classification used to be able to "just do it"; it can't do that in Oreo and beyond. It can try to stick a foreground service (for example) but unless you authorize it specifically to do so even though it would otherwise post a notification it still can't make the service "stick." In addition even repeating alarms (which used to reliably allow an intent to be sent and thus an app to be "woken" or "activated") will be deferred despite the app's attempt to make it otherwise, UNLESS you specifically exempt the app from battery management.
Further there are rules for Oreo-and-beyond for when you can include that privilege in the manifest (allowing the app to request it from the user in the first place) and if you submit an app that has that privilege in the manifest you have to justify why it's there. If what the app wants to do CAN be served by FCM (see below) it MUST use FCM, or Google won't approve it.
The exception is that an app can register for what is called "Firebase" (FCM) which is a cloud-based notification system, and that goes through the system itself. However, high priority notifications (which can wake the device out of doze), if abused, now become something Google can detect since ALL such notifications go through their systems and they have fairly explicit developer rules on when it's legit to use those (e.g. for delivery of instant messages) and when it's not (e.g. other sorts of content, for the most part.)
Same deal with notifications -- an app HAS TO register a channel to post them (if it doesn't then NOTHING goes through!) and once it does YOU have control over whether they are delivered, if vibrate works, if sound works, etc -- and the app CANNOT change it back. All it can do is send you to the system's app notification page (where you can make the changes) but it can't force them -- if you turn 'em off then they're off, period. An app can register multiple notification channels if it has multiple "types" or "classifications" of things to notify you about, but in any event you can always modify or shut it off and the app writer cannot override what you've done.