Those little popups may reveal location, device details, IP address, and more
"App developers can encrypt these messages when they're stored (in transit they're protected by TLS) but the associated metadata – the app receiving the notification, the time stamp, and network details – is not encrypted."
Another quote from the article: "The data that is required to 'turn on any push notification service' is quite invasive and can unexpectedly reveal/track your location/store your movement with a third-party marketing company or one of the app stores, which is merely a court order or subpoena away from potentially exposing those personal details."