Even connected to wifi a cell phone canstill use the wireless network. Even airplane more won't actually stop your phone from connecting anymore. GPS data can also be transmitted in the background over wifi back to apple/google and/or the device manufacturer.

If they really wanted to push this they could do it directly in the baseband chipset and bypass the OS entirely when deciding to lock down the device to some kind of "travel mode" with limited functionality (such as no texting or no browser)

Not that I'm advocating for that sort of thing, but it's good to keep in mind that we don't really own the cellular devices we pay for and that even in the rare case we have root we can't stop them from doing what they want to our devices as long as they control the closed hardware.