Is this necessary? What happens if we omit it? Is this what you were referring to in the diff description?

What this differential does is to structure rescind as if it was a regular notification. This is required to launch NotificationService to process it.

I was testing it without artificial body property and it worked. However I once came across this article by apple: https://developer.apple.com/documentation/usernotifications/modifying_content_in_newly_delivered_notifications?language=objc, they describe a case of decrypting encrypted notification in NSE. The payload for notification contains body property with artificial content in aps dictionary even though it is not used. Moreover this doc states:

The payload must include an alert dictionary with title, subtitle, or body information.

So I am afraid that sending notifications with alert field without anything that could have been displayed as an alert to the user might be considered as a violation for Apple.

Your reasoning makes sense, but I think you are setting alert to a string, rather than a dictionary here. See the code here

It is specified here, that alert might be string or dictionary: https://developer.apple.com/library/archive/documentation/NetworkingInternet/Conceptual/RemoteNotificationsPG/CreatingtheNotificationPayload.html. I was thinking more general: Apple wants some content that can potentially be displayed to the user.

But the doc you linked says "The payload must include an alert dictionary with title, subtitle, or body information."

It doesn't appear that a string qualifies as a dictionary?

I think it'd be fine to remove this entirely if it works without it. But if we're including it just to match the docs, we should make sure we're actually matching the docs.