I can attest that the soya-based wiring can be a nightmare in some locales. A family member bought a new Acura in the 2010s and the car was a constant target of the local rat population that would climb into the engine bay and fill up on wiring insulation like it was gummy candy. He had the wiring harnesses replaced several times but ultimately gave up and sold it after the final repair.

It seems like they could incorporate capsaicin and other deterrents into the polymers, but that probably isn't pleasant for individuals involved in the production process.

FWIW, it's a myth. Confirmation bias continues to drive it. Rats love wires no matter what they are made of, they happily chew up petroleum-based plastics too.