This is well known in the op-sec communities. iOS and Android notifications route through their servers and can be stored indefinitely (ie especially under a court order) You need to disable the content previews if you want to be secure. But even the notification metadata can be quite valuable to law enforcement (who is messaging you, what time of day, etc.)

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.