I didn't get this message but I saw a change in the car map that shows all the superchager stations. Yesterday, it still showed the cost (ie 26 cents per KWatt) at each station. Now, the charging cost disappeared and it just shows the number of stalls available and the idle cost....just like my other 2016 model s. I got this 2018 inventory car last week (ordered it before the Sept 16 deadline with my own referral code) so I have free supercharging.
On my Tesla account, it now shows I get free supercharging for both of my Tesla's. With my 2016 model, it says the free supercharging is transferable to the next owner. And for my 2018 model, it's not transferable if I sell the car.