I'd half expect the infotainment system to just refuse to work at all if this was done in a simple way (like damaging or removing the modem). Then no stereo at all or Apple CarPlay or Android Auto. I suspect the "right" way to do this would be to ground the LTE antenna or faraday cage it or something like that.

I'd expect cars to still work in remote areas or underground parking garages where there isn't any cell service, and this would simulate that.

Since I don't see the normal reply to this completely predictable "you can just disable the hardware," I'll drop a TL;DR version of it:

No you can't, not without voiding your warranty.