What happens if a deleted or unregistered user re-enables push notifications?

If a device is deleted from your database or unregistered, the app can register the user again depending on how they were removed.

If you delete a device from your audience database, the SDK automatically sends a registration request the next time the user opens the app. If the user has push permissions turned on in their device settings, they are registered again as an active subscriber.
If you unregister a device using the /unregisterDevice API, the system deactivates the device token. However, the SDK will still try to register the user the next time they open the app. To stop this automatic registration, you must write custom code in your app to block the process.