Depends on your definition of opt-in. The app has to 'sign up' with the push notification service of the plattform to receive push notifications. But most apps

Depends on your definition of opt-in. The app has to 'sign up' with the push notification service of the plattform to receive push notifications. But most apps do this by default. I guess that in most cases this makes sense. most people who install an instant messaging app want to be notified of new messages.