I’m not sure how E2EE would work for notifications. The ISV sends the notification but the OS displays it. I suppose some key exchange between each ISV and each device?
https://support.pushover.net/i46-are-messages-notifications-...
My understanding is that Pushover encrypts the message with your device's public key (from the app) and delivers it through Apple/Google. On iOS you need a special permission from Apple to "filter" push notifications and this is the hook they use to capture and decrypt it before displaying it on your phone