At least On-Star is optional, now.
Sort of. Hard to find higher end GM vehicles without it, and even if you don't keep the subscription, the hardware is still there and can be reactivated by the company without your consent
You don't have to buy a GM car. If you do buy a GM car, paying for it so YOU can use it is optional. But, they can always turn it on if THEY feel like it, so unless you disable it (unplug the transmitter, remove the antenna cable, etc.) it's ready to help you, or to "help" you by monitoring you.