OnStar reports location and speed data to the car manufacturer. Sometimes they will sell this data to insurance companies to raise your premium, as several news stores pointed out a few weeks ago. I couldn't really find an advantage to OnStar, (I have my phone to call emergency services) so I disabled it by pulling it's fuse.
For my 2019 bolt, it's f31 in the instrument panel fuse box, just down and to the left of the steering wheel. The fuse box cover comes off when you pull it hard from the bottom.
I was able to find which fuse went to OnStar in the owners manual and labeled on the inside of the fuse box cover. You should be able to find it for your model car there too if it uses OnStar.
I did have the casualty of my speaker for calls and texts. I'm not able to use it right now. I'll see if I can dig in and reconnect it somehow, but we'll see.
Who knows that other into they're snitching back to GM, or what they could do in the future, so I recommend disconnecting it. Good luck!
I looked at some wiring diagrams and forums; the wifi, Bluetooth, and OnStar are in one module. The module is able to be taken apart with a torx bit, I can't tell if it is a security torx. You would need some understanding of how electronics work to strategically break it so OnStar can't connect. There is the potential that no antenna can cause damage to the module, so be aware of that.
You can make a dummy antenna to plug in to make it so the module can't connect and lessen the likelihood of the entire module failing. There appears to be some ability for the car to phone home but I didn't find an answer if it was connecting to the phone, grabbing wifi somewhere, or if the dummy antenna still had enough connectivity.