That's probably true, but I don't live in some places. I have a door with a knocker and a doorbell. Calling/texting me is so damn annoying and pointless. You knock/ring, I get up and answer the door. You text/ring, I might ignore it or at least it could take time to go to where it might be sitting other than in my hand.
If you live in one of these "some places", I doubt the landlord is going to be very happy about people plastering various QR codes on their property, especially if "some place" is a multi-tennant property which typically already have a system in place
The ‘system in place’ in question: two non-functional doorbells, a third wired up to your housemate’s room, and a fourth which only works on Tuesdays (nobody in the house knows why). :)
Mailman, flyers, delivery not for me, neighborhood kids pushing wrong buttons or just fooling around.
If I have delivery scheduled or there is scheduled maintenance for building I will connect it for that day. Even delivery guys sms/call me when they are in area to check if they should drop by with parcel anyway.
People like friends/family don’t drop by without calling sending a message. There is no unscheduled visits since I was teenager.
Some renting situations make solving those problems difficult, annoying, or impossible.
But yeah, generally a doorbell is a pretty reliable tool.