Remote shutoff on vehicles, sadly, isn't a new phenomenon.
I'm not sure how many years ago it was, but a locally well-known icon had retired from one career to be a new car salesman and later a used car salesman. During the latter, period, it came to light that anyone buying his used cars under subprime lending conditions had a remote shutoff switch put on their cars while they were paying off their loan so that the cars could be disabled if the buyer defaulted on payments.
As a result, the dealer faced lawsuits from people who either: (1) had their cars shut down at inopportune times (such as in the middle of driving on a busy highway), or (2) made their payments timely only to have the dealership shut their car off because their database incorrectly listed the payment(s) as delinquent. The hit to his reputation was such that his dealership closed and he seems to have faded into obscurity.
TL;DR - The ability to remotely disable cars already exists and, yes, it's the shit heap you might expect it to be in terms of how its executed.
With automakers doing what they can to make DIY repairs difficult if not outright impossible for knowledgeable normies, I can see them doing whatever it takes to prevent people from easily converting their existing vehicles to electric. With the trend that corporations see people as little more than consumers and dollar signs, automakers won't want to see people do anything that deprives them of a revenue stream.