On some cars you can pull a fuse that goes to the cellular modem, like many Fords at least up to around ‘21-‘23.
On others you may need to physically remove the modem if the fuse is going to more components.
Not sure about GPS but yes, some cars lose features when you do this. Can’t believe automakers dropped remote start from the fob just to push people to pay for a monthly subscription.
On my 2023 Subaru, the "Telematics" device is a small box behind the dash on top of the radio. I simply unplugged the GPS and Cellular cables attached to this device. Of course, now I can't use the OnStar-like emergency button anymore to call for help in an accident.
I assume the cars with active telemetry have a SIM card of some description. You could remove it/disable it but you'd lose app access and remote control features. I know of people doing this in Toyotas.
On GM cars it's pretty easy to disconnect the OnStar antenna. I don't know what that does to the built-in GPS, but my answer is only buy a car with CarPlay.
I've found on some cars (i.e. Jeeps), disconnecting the antenna is insufficient. It just lowers the range of the cellular modem. If you get close enough to a cell tower it'll still find a connection to the mothership.
In my case I had to open the radio and physically remove the cellular modem. Which was thankfully on its own removable module.
Why do you need CarPlay or Android Auto? I'm using my car without any of them and it's perfectly fine. I get calls with Bluetooth and I can play music with Bluetooth too, both from a Samsung and an iPhone. I navigate by looking at my phone on a holder close to the wheel. The screen of the car is further to the right.
I have a different use case. I use voice controls to reply to text messages.
And it has to work with iMessage destinations that are not a phone number. I am unaware of any built-in infotainment system that can handle that. Tesla certainly cannot, as one data point.
Trying to get voice control to work reliably over bluetooth has never been anything but a laggy mess for me. A well implemented CarPlay implementation (even GM...) is way better.
Or does this break other things, like the built-in GPS and similar?