Sorry, not familiar with Android core. For your example, how would it work without proprietary technologies? AFAIK, Android uses GCM for notifications. Adding integrations for it don't seem to make much sense in core open source Android as that will make you tied to Google's ecosystem.
Both are serving the same purpose, for different apps. Even before we get to network connections and servers (the things that might make sense to be proprietary), without Google's ecosystem the local side has a bad UX and requires hacks.
That is just because they are not using something like https://bubu1.eu/openpush/ and instead implementing their own push services. This is not a problem with Android but with the apps.
I was speaking about _local_ notifications, not push: some clock / reminder apps never fire notifications or way too late. I think it's because the OS aggressively shutdown apps running in background.
It doesn’t have to do anything, but if it doesn’t support what are considered standard features of a phone today in an open manner, it’s hard to take AOSP seriously as an open platform.
Also worth noting that when someone does build on top of AOSP, they are at the mercy of Google’s roadmap for the closed versions of Android.
The fact that this is possible is something I have commented about on this very forum for years. It is trivial for the Amazons and Huaweis of the world to build an API like this that supports Google's FCM, Amazon's ADM, etc. and provide a library for developers to use that will let them easily deploy push-enabled apps on devices across all ecosystems.
It is already usable. Nobody would claim that a Linux distro isn't open just because it doesn't have a built-in push API. You can build one on top for other apps to use, and you can do the same on Android. This is notably different from iOS.
Is there even a functioning darwin build for Big Sur??