I'm curious if cars would be bricked if they couldn’t call home, or if you could selectively allow certain messages through.

I can't speak for every car but at least Teslas do not mind being offline. You cannot control which messages they send because they connect via a VPN to the mothership. So it's an all or nothing kinda deal.

You can also pretty easily remove the SIM card on older models with just a few screws. Newer ones use eSIMs, never looked into how to get rid of that one but I assume it is more complicated.

Your comment makes me wonder if one could get around AT by installing faraday cages around where the chips are.

The antennas are usually external, mounted somewhere else in the car and can be unplugged. Never checked if it can still get a signal without the antenna though.

edit: Also, the PCB itself is mounted inside a faraday cage because the entire thing sits inside of RF shielding.