I can see some potential for people with hearing loss. I've seen apartments and houses of such folks, with some small dedicated bulbs or even normal ceiling lights flashing furiously on all over the house upon someone ringing the doorbell.
A doorbell signaling smartphones, making smart wrist watches vibrate etc, could be reasonable in such cases. But it could be still a plain uncomplicated button next to the door.
A "smart" doorbell that can notify you in various ways makes sense - even without hearing loss you may not always be able to hear the doorbell (headphones).
Expecting guests and random strangers to go out of their way to accomodate that is not ideal. I don't know how I would react if I had to scan a QR code to ring a doorbell but chances are I would leave.
