There already was a post like this this year but now my answer is "a standardized push notification system (most likely federated) that's actually possible to be implemented in a user friendly way". Google doesn't want to encrypt theirs afaik and apparently some people are concerned about the traditional "every app is responsible for its own notifications" approach consuming much more battery, even though I didn't notice it myself (I guess it's possible if you have 50+ apps installed but it's not something that should be a thing in the first place).