or modified to prevent all the telematics from flowing back to them
A simple hardware modification is all you need there.
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Find the antenna for the on-board modem.
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Disconnect the cable that leads between that antenna and the car's brain.
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If necessary (if the car gives annoying/disabling errors after step #2), connect a high-ohm resistor between that antenna connector and the car's ground.
After that, the car will forever think that it's outside of signal range. And since operating while outside of cell service range is something even the most modern of cars still need to do, all of the car's essential functions will still work. You'll lose any internet connectivity features, of course. But in exchange, you'll have completely and permanently disabled any possibility of remote telemetry.