Many cars have the modem / LTE as a module that can be disabled. On my car, a 2018 Chevy, the LTE antenna plugs into a networking module in the footwell. I bought an antenna cable and dummy load resistor on Amazon for ~$10 and simply unplugged the LTE antenna in the car and plugged in the dummy antenna. That disables all cell network connectivity but doesn’t effect things like Bluetooth. It’s also easily reversible if someone wanted to do that for some reason.

You can probably Google something like “year make model disable LTE” or “disable telemetry” to get an idea of how hard it would be on your car. Some make it very cheap / easy and some make it very difficult.

In many cars, the modem that phones home with your data may be a separate unit. Depending on your make and model, you can sometimes buy cable adapters that will connect power to everything except that subsystem. The term to search for is a “DCM bypass plug”.

Buy an old car

(please quietly make the “buy an old car instead” jokes to yourself)

That’s not a joke, it’s sincere advice. My newest car is a 2008 (I think) model. I’d love to have an EV, but I don’t because I haven’t figured out how to have one that isn’t infected by “telemetry” and other enshittification. I have hopes for the Slate Truck, but it doesn’t actually exist yet, so I wait.

(Also, yes, as a mod of the place I’d be remiss if I didn’t second the other reply and suggest adopting a non-car-dependent lifestyle to begin with.)