As much as I hate cybertrucks and the sort of people who would buy them, I hate the diminishing of our property rights even more and thus have to object to the notion of manufacturers punishing people for exercising their right to modify their property. Frankly, the “glorified cruise control” ought to be Free Software to begin with so that the owner could modify it as well, to be compatible with the horns. (The owner would of course be legally responsible for the result, but that goes without saying since it’s how property law has worked for hundreds of years.)

What should happen is that if the modifications make the car unsafe (whether because of the cruise control, the horns themselves failing pedestrian safety standards, or otherwise), the government prohibits the owner from driving it on public roads. But the keys are that it would be a restriction by government (not the manufacturer) of how the thing could be used affecting the public (not whether it’s allowed to exist at all).