> Kias have a “Massachusetts mode” flag hidden behind a service menu (that needs a dealer code) that disables telematics at the owner’s request.

I would be very concerned that the flag just continues to submit your data but with a "telematics disabled" bit set on it. This is absolutely how location privacy is implemented in some devices. Moreover, even if it is effective it could be remotely reset including accidentally as part of an update.

Better than not setting it, I suppose! :)