This can be done on older versions as well but the app has to declare itself as an Accessibility Service, which is a massive hack. Google are fixing that by mirroring that specific use case in a separate service.
One of the other advantages of this new API functionality seems to be that it will also allow apps to dismiss notifications, or detect when the user has dismissed a notification. This will allow notifications to be kept in sync between the system and an app (or on a linked device like a smartwatch), which is a very nice addition.