It’s intended as a consumer-friendly way to increase capacity in dense areas (like a sports stadium or mall) where the carrier’s cell towers don’t have enough capacity.
Wifi offloading is not new. AT&T helped invent these standards back in ~2009 when their network was getting crushed by massive increases in traffic as iPhone usage took off.
WiFi offload networks are configured as “Managed Networks” which are lower priority than any user-selected networks. You can disable them by turning off “auto-join”. (Also these WiFi offload networks are secure; you can’t spoof them).
However it appears that the original poster’s carrier (presumably Xfinity Mobile or Spectrum Mobile) has done something new - they’ve disabled the user’s ability to turn off “auto-join” on iOS. Some overzealous team is trying to lower their cellular costs. That’s because both Comcast and Spectrum rent capacity on Verizon Wireless towers, but their MVNO cellular service is not profitable unless their customers are using the cable company’s own WiFi fairly often.
However this (disabling “auto-join”) is a dumb move. It’s obviously problematic for users whose neighbors are broadcasting the [Xfinity WiFi or Spectrum Mobile?] SSID.
To my knowledge, no major carrier does this. If you’re on AT&T, T-Mobile, or Verizon, the “managed offload networks” can be easily disabled. And the major carriers are using higher-quality commercial WiFi networks for offload, not random home cable modems.