This is really easy to do now on Android. Just long press the notification and you can just flip the notification toggle. The real problem though is people passively accepting their phones defaults. Users need to be educated somehow that it's in their best interest to actively curate their notifications, or better yet, put the onus on Apple/Google. What if the default for the first run of an app was to prompt you if you want notifications or not? That would certainly get rid of some cruft.