If that's the case, then I'd hold this as a different threat model than not being able to turn off WiFi and Cellular.
Very curious if an iPhone or iPad with all accessible settings off, including for NFC turning off Apple Pay, NFC tag reading, etc., leaving only this background NFC on, if there are still persistent identifiers being broadcast.
I could be wrong, but on a lot of mobile SOCs all of the modems are in the same chip as the CPU. I think you would have better luck removing the connection to the antenna
iPhones will transmit bluetooth beacons even if turned off. Fortunately the battery goes completely flat after a couple of weeks or so and then they no longer do. Unfortunately this is not very healthy for the battery.
the only way to disable any transmissions is to turn off the device