Idea: buy a car with an outdated (3G) modem and simply don't replace it.
I don't think that really exists -- the cars either don't have the connectivity, or it's relatively modern (4G+). For the older cars we had to use dongles, which in some cases were even 2G.
Are you sure? I believe there was a big recall for at least one model of car during COVID due to the chip shortage + impending 3G shutdown.