I believe Vitor was refering for all apps individually.

Anyways I recall thinking on an idividual notidications app while working on Nostros. It doesn't have to be centralized somewhere, it's just a listener in your phone that pings your notifications center where there is a match. 

 yes, but it obviously needs to be a protocol, and a new NIP is required so you don't have to set DND on every device