FWIW WhatsApp has this builtin, at least at the device level. If you change a device, the other person is notified. If they steal it, unlock it, clone the user

FWIW WhatsApp has this builtin, at least at the device level. If you change a device, the other person is notified. If they steal it, unlock it, clone the user's voice before the phone is marked as lost, then yeah still bypassable (but so would be a device-based QR code)