The software that enables features has been in the gray zone lately. That includes FSD, EAP, Premium Data Fee Exemption, and of course Unlimited Supercharging.
It is great that if all those features are transferred then there would be no complaints.
Except, if they are not (maybe because of a Tesla's error), then how does a new current owner of a used car prove that?
If you buy a used car from Tesla directly, it would list whether the car has Unlimited Supercharging or not. Its "Motor Vehicle Purchase Agreement" would indicate whether you bought Unlimited Supercharging or not.
If by mistake, your current used car Unlimited Supercharging is taken away, how can you prove to the court that your car should have that? The court routinely recognizes a standard legal contract that's called "Motor Vehicle Purchase Agreement". If the feature is not listed as paid there, you might argue that there's a verbal agreement, a written e-mail... but they are not "Motor Vehicle Purchase Agreement". Because if those e-mail and salespeople promises are good, they would be reflected and documented on the "Motor Vehicle Purchase Agreement".
You can still argue that the verbal/e-mail agreement should be legal but that's much harder for your case (Tesla could just fire whoever did those verbal/e-mail agreements citing those are contrary to the legal practice of the company because no single employee has the authority to create a para-"Motor Vehicle Purchase Agreement".
In summary, if you want to take a chance then just use your faith to believe that your Unlimited Supercharging will be transferred without the appropriate documentation.
If that were me, I would ask for the past or current "Motor Vehicle Purchase Agreement" that specifically documents that free feature.