So this is maybe (probably?) unrelated, but when touching down on an airplane, I always start getting push notifications even before joining the local WiFi etc. What's going on there? Am I imagining this or does it actually happen?
It's usually not that those specific services are whitelisted, but that a number of hostnames have to be whitelisted for the user experience. Sometimes, those hostnames include those types of services, depending on the phone, OS, carrier, etc.
Can someone who has dived into the code on iOS/Darwin or Android/Linux kernel verify this?
If no: i would say no. But if you get an alert popup, it can be mechanism like mobile sending generic alerts for tornado and stuff