Also standard requirement on govt mobile devices to disable notifications. Mattermost provides this option at the server level to block notifications entirely for ios/android devices.
Signal's server-side push notifications only contain a "wakeup" message. The actual message popup is displayed after decrypting the message contents locally on the device. Of the things you mentioned, only the time of notification is visible to Apple/Google.
Why are app notifications not part of app data that gets deleted on uninstall???
Most notifications are sent by backend servers straight to Apple/Google
So in a sense it is part of the application, especially if you're a small entity with a single app (as opposed to large entities like Facebook where you have dozens of applications under a complex hierarchy of developer and application certificates).
I can understand why things are done this way. It helps to avoid abuse and spam as there's no way to inject notifications without strict accountability. But it does kind of suck. To fully self-host IM, you need to build, sign, and distribute the client yourself, as well as run a notification gateway with the appropriate credentials. And I'm not aware of any plug-and-play open source solutions for the gateway, at least not for XMPP. (I could be mistaken, though.) Maybe Matrix servers have it builtin, but I wouldn't be surprised if they don't, especially the reference implementation, as this complexity provides a moat for monetization.
fwiw, as far as I can remember, the signal foundation's position has always been "once someone has physical access to your device, all bets are off."
Show previews is set to Never (Default).
> Notification Center shows your notifications history, allowing you to scroll back and see what you've missed.
https://support.apple.com/en-ca/108781
Note that although Android has a similar "notification history" feature, it's disabled by default and requires opt-in.
People would generally claim "lazyness", as that is the Apple way. Why fix code when you can just sell new phones?
The actual answer is plausible deniability. Closed source software often leaks metadata in hard to discover ways so governments can deprive citizens of their rights under the law, and then claim "whoops, we didn't clean up correctly, our bad!".
Apple, like every other major tech company, goes along with it when nudged in the right direction.
