Whenever a new device (AppleID/phone number) is associated with Messages/iMessage, a notification appears on other devices saying something like "Your phone num

Whenever a new device (AppleID/phone number) is associated with Messages/iMessage, a notification appears on other devices saying something like "Your phone number is now being used for iMessage on <device>", along with some more text. Apple also sends an email about that to the email address associated with the iCloud account. So I don't understand what you mean by "it would be easy for Apple to simply add a new device without showing you". If that refers to Apple circumventing this kind of warning for specific users for some reason, then I guess other popular messaging platforms/apps cannot be trusted either.