I know I’m a few days late to the discussion but I thought the difference in iOS 13 was that Apple were changing the API so that work (like decryption) can only be done for user visible notifications but not for invisible ones. The problem for encrypted messaging comes from the fact that you don’t know if the notification needs to be user visible until after you decrypt it. Previously encrypted messaging apps always used non user visible push notifications and intercepted them decrypted them in the background and changed to visible as necessary. For instance some messengers sent “... is typing” signals over the push messaging system but wouldn’t show those to the user but would show a notification when a message was received. The new API doesn’t allow that because either all the notifications need to be visible to be decrypted (so you get spammed with visible notifications you don’t want to see) or your invisible push messages don’t get decrypted until you open the app and you don’t get message notifications.

The only way around this is for the server sending the notifications to know whether the message being sent needs to be displayed which means the server needs to know what the encrypted data contains. This is seen as problematic because it undermines the end to end encryption.