[fy20]

I remember something like this happening nearly a decade ago with an iPhone 5S. I was at a large mall I visited often and saw I was connected to a WiFi network I hadn't used before.

The mall had WiFi but there was a portal which required SMS authentication and was time limited (the same as every other hotspot, it was rules of the country), so I didn't bother using it on my phone. Plus the carrier had a modern LTE deployement, where I'd often get over 50mbit download speeds - which was faster than my home internet. The network was named something like "<carrier> offload" so I assumed they had a kind of WiFi deployment to limit cell tower load, and it was added by the carrier settings profile.

I can't remember if I was able to disable or delete the network (it worked, so I didn't care). I'm wondering if this feature has been there for a while, but OPs ISP has only just decided to use it (I imagine some exec had an OKR to increase adoption of their public WiFi hotspots).