When I bought my used car from Tesla, all cars listed for sale on their site that were pre 2017 models came with lifetime unlimited supercharging and it said so on the pre-owned site. Now none of them say it. The problem is all Model S and X before the policy change came with this feature and this feature was listed on the sticker and build.
Tesla does not want to offer this anymore, but I don't know how they can't since it is in writing. Now obtaining that written information now is the difficult part. This is no different from them pulling premium connectivity from previously owned cars. They tried, but realized they couldn't since "trial" wasn't written in any contract with previously owned cars.
This is something that should stay with your 2016 model year purchase, but it may be a fight to get it just like premium connectivity was. The fight will benefit all though so I'd say it's worth it.
I know it's written that all cars prior to the supercharging policy change came with free supercharging for the life of the car. Elon Musk, while he was CEO, is even quoted as saying it in an interview.