You can already turn off notifications for an app in Android.

Making it a permission just makes it default -off rather than default -on

There are alao plenty of us who just have notifications set to vibration mode without any sound full-time. The only phone alert in my case that makes sound is the alarm. Unless i put my phone on some hard surface or literally next to the bed, there is no way i will hear vibrating notifications, even when awake.

In general, I found vibration-only to be pretty much the default for a lot of people I know (though I am sharply aware that this is just anecdata and doesn't say much about the real state of things). I tried googling whether there was some breakdown of notification vibration vs sound smartphone users, but, sadly, couldn't find much.