Originally Posted by maalloc

Data point from a few days ago:
On a connection flight (MEX-GDL) after a long-haul coming from Europe, they requested everyone to get the QR which, as you have seen, requires online registration from a smartphone.
Let's just say the pax arriving there, some of them without a local data plan on their mobile yet, were less than happy and tried to talk their way out of it. But it was a hard requirement for the transfer. It does seem that pax leaving airport right after landing in MEX were not asked for the QR, though. Your luck may vary.