Worse, iPhones immediately start backing up to iCloud when set up for a new user - the only way to keep your network passwords and all manner of other stuff from hitting iCloud servers is to set the phone up with no network connection or even a SIM card installed.
Did I mention there's no longer a SIM slot, so you can't even control that?
And that iPhones by default if they detect a 'weak' wifi network will switch to cellular, so you can't connect the phone to a sandboxed wifi network?
You shouldn't have to put your phone in a faraday cage to keep it from uploading plaintext versions of your private communications and network passwords.