What about feature phones?
If the tech requires an app then that excludes all the millions of feature phone users.
If it requires bluetooth then you need to tell folks to turn that function on, assuming their phone includes it. If the person is unable or unwilling to enable bluetooth, that kind of drops an extra large hurdle in your path.
If it requires wifi then you're back in the same boat as with bluetooth. If the phone doesn't have it or the person is unwilling to turn it on, there's a giant hurdle in your way to track the person.
China is a large market for feature phones to the tune of millions of users, and anywhere that uses 2G/3G as their primary cellular network infrastructure is another high concentration for folks that can't afford a smartphone. So what do you do if the phone the person has in hand is unable to do any of the techno-gymnastics you require in order to do the job? Do you buy them a smartphone & set it up for them so it/they can be tracked?
"Hey you! Turn on your smartphone & scan this code!" Sorry, it's a feature phone & doesn't run the scanner app. "What kind of neanderthal ARE you? No smartphone? HEATHEN!"