Does blocking notifications at the system level stop data usage?
So does blocking notifications stop data usage, or not?


It does not. A notification as stated in Notifications Overview is simply:


  a message that Android displays outside your app's UI to provide the user with reminders, communication from other people, or other timely information from your app


When you are connected, your apps will connect to servers to receive messages and communicate them to you. Blocking the notifications does not block the apps from receiving the messages, but from showing them to you.