That breaks wifi (though wifi is enough to leak decent location data, unless you’re using a vpn, and somehow blocking location service).

I wonder how hard it is to open a modern iphone and cut the relevant antenna traces on the pcb.

There are youtube videos on iDevice wifi modifications. You can also use a wired ethernet adapter on iOS devices via the lightning port.

at that point, what's the point of using a phone?

There are many offline apps/data which can be synced via wired ethernet, including maps, audio, music, video, photos, books, notes, sound/light meters, compass and special purpose add-on devices.

exactly. i think the issue with cell with people is that it can track your movements and most people always have it on them and on.

Surgery: No GPS antenna, no GPS location.