I've only seen it once.

I was doing some work in a small-ish county jail/sheriff's office in the States. As part of that work, I needed some Internet access.

Because jail (thick, reinforced walls and lots of steel) the cell phone coverage was basically shit -- otherwise I'd have just used my phone like I would normally have done approximately anywhere else.

It was a fun dance: Requesting access via wifi, getting sent a code via SMS, and then going outside, turning off wifi to establish an actually-working network connection, retrieving the code (yay Google Voice), and then going back inside, turning on wifi, entering the code, and actually using it.

There was some other detail (perhaps relating to very short timeouts or re-registration issues or MAC randomization) at some stage of the operation that seemed extra-insulting, but my mind has forgotten whatever it was.

I have no idea what this song and dance was intended to provide, prevent, or enforce.

It's required by law in some countries, and it leads to some very funny chicken-and-egg situations with airport WiFi.

Istanbul Airport added a workaround: a physical passport scanner that stores your info and generates a code as an alternative to SMS verification. The whole thing just feels like a VPN ad.

There's at least one country with laws that say you have to keep track of national ID numbers (and times) if you want to provide wifi service.

Never seen it in the US, but it was fairly common when I was on vacation in Europe

A now deleted comment reminded me that this is mostly for in-flight wifi where it makes much more sense. Mostly no SMS there either.