In-reply-to » Another thing that doesn’t work anymore after blocking network traffic from my Android phone: Some push notifications. I haven’t done any app development, but I know notifications on phones are indeed dependent on cloud services run by the OS vendor which talk to servers run by the app vendor on your behalf. This is supposedly better on battery life, but it conveniently lets your OS vendor read all your notifications.

Mobile XMPP clients usually implement notifications using XEP-0537 and it goes like this:

Your XMPP server -> Client vendor's notification server -> Client OS notification server -> User's device

It’s not end-to-end encrypted so servers will usually just send a dummy message through (You received a message from juliet@capulet.lit!) so you have to open the app to see the (hopefully) encrypted message.
It’s a similar flow on both iOS and Android and I assume Matrix clients work the same way.

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