This is up to app developers to implement themselves. The mechanism Apple provides for this is UNNotificationServiceExtension, which allows an app to step in before a notification is delivered so that it can do things like decrypt it.
Encryption should have been the default in the official framework/API to begin with. But I assume E2EE wasn't a concern for Apple/Google back then, and now it's to late to change everything without a lot of pain.
Apple previously didn't have any problems making breaking changes to their stuff, they even switched their whole CPU architecture and said "deal with it". If E2EE was really a concern to Apple, they'd implement it.
Push notifications just have the convenience of even technical people often not knowing that most of them are routed directly through Apple/Google. So how should your average user know? It's free data and nobody complains about it how they would with messengers eg
> Apple previously didn't have any problems making breaking changes to their stuff, they even switched their whole CPU architecture and said "deal with it"
What? Apple put a tonne of effort into continuity between arches. You think Rosetta is them saying “deal with it”?
> Apple put a tonne of effort into continuity between arches.
Still can't run windows on an M* mac, can you? Why? Because there is literally no documentation for the CPU and they didn't really follow any standards. They just built some shit that literally only Marcan probably knows how it actually works. Even the people who made it probably don't know how it works, only how it is supposed to work.