Cross posting from the other thread:
I ended up calling and having OnStar cancel my "trial" in my 2018 era truck in 2019, to the point I instructed them to disable the OnStar lights on the mirror and everything. This was a task as they're very annoying and persistent...
Once that was done I originally pulled the daughterboard housing the entire modem but this bricked my compass and GPS. To keep those working I ended up altering the modem itself with a soldering iron per the pdf posted here:
https://www.chevybolt.org/threads/internet-without-onstar-wi...
The guy there on that forum did so in an effort to use your own sim with the HMI, but using 0 ohm resistors or just solder bridging the relevant connections is enough to prevent my vehicle from getting online, but leaves the GPS intact. You can test by trying to make phone/onstar/data related actions - it just won't work.
Another good measure is to remove one of the antenna leads, I forget which one is cellular primary, but the other is more GPS/aux. Doing it all this way allows you to still connect your vehicle to wifi should you so desire to, for updates and such.
There are other secrets and tricks to these things, including a git repo I cloned that is now gone from the public internet, where a guy figured out how to solder an EMMC reader to a few points on the mainboard to get and modify the filesystem.
Edit: here's a mirror
https://repo.or.cz/bosch_hmi_hacking.git
They really go out of their way to keep you from owning your vehicle.
Also bought a device that lets me plug HDMI inputs into the truck, unfortunately some kind of refresh rate or resolution issue prevents it from working properly.
As an aside, if you bought and financed through GM, they may have attached a secondary GPS/cell tracker to your OBD2 port, and used sticky tape to hide it somewhere - make sure to remove that once you're not financed through them as well (it is illegal to do so I believe before using alternative financing or paying it off).