Or in the case of UK users, provide an absolutely absurd amount of PII to demonstrate that you're of age.
I have Monster Girl Island: Prologue from back in the day, that's fine. Sits in my games list causing no trouble. Can I look at the release notes for updates? Not without entering credit card details. Makes perfect sense.

I wrote one of my own from scratch, back in the day. More to practice my algorithm coding skills than anything else, make sure that I could. Not very difficult - easier than barcodes, in a way.
The thing that I found most interesting was that it uses the same Reed-Solomon error correcting code as CDs and DVDs, and for the same reason. Those codes guarantee that you don't get too many 1s or 0s in a row. That would cause difficulties with laser tracking in a disc player, or big confusing areas of white or black in a QR code.
The on-off-on-off pattern that joins the inside edge of the three squares isn't usually that obvious either, but when reading it, makes it quite easy to decide how 'big the boxes' are. You can store a very long piece of text in a QR code, although the pattern gets very finely detailed after a while.