The (archived) Local and Remote Notifications Programming Guide contains the following:
To support a background update notification, make sure that the payload’s aps dictionary includes the content-available key with a value of 1. If there are user-visible updates that go along with the background update, you can set the alert, sound, or badge keys in the aps dictionary, as appropriate.
The current documentation however reads:
To send a background notification, create a remote notification with an aps dictionary that includes only the content-available key, as shown in the sample code below. You may include custom keys in the payload, but the aps dictionary must not contain any keys that would trigger user interactions.
What caused this change and why is no longer supported to send additional keys (e.g. an alert dictionary) in the aps dictionary of a background notification?