Supercharging was free for life, then it wasn't, then it was, now it is again, but just for a limited time.
Tesla's Supercharging policies have been all over the map, making it difficult to follow the current practice that's in place today.

That's after several free, not free changes and deadline extensions in the past.
Confusing? Absolutely. Well, prepare to be confused once again. Remember Tesla's recent "free for life" Supercharging announcement? It read:
"You have free, unlimited Supercharging, including your current Tesla and any new Model S or Model X you purchase. If you choose to sell your current Tesla, free Supercharging will not transfer to the next owner. Also, up to five friends you refer will also receive free Supercharging. Any Supercharging payments made have been automatically refunded. Read our Supercharging terms.”
“We heard from our customers that free, unlimited Supercharging was one of their favorite reasons to refer a friend to buy a Tesla, so we’re bringing it back for Model S and Model X owners through our referral program, which we update periodically. Beginning today, existing owners can give free, unlimited Supercharging to up to five friends by sharing their referral code, and all existing Tesla owners who purchase a new Model S or Model X will receive free, unlimited Supercharging too.”
But now it's been changed slightly to include a deadline for purchase. It now reads:
“You have free, unlimited Supercharging for your current Tesla vehicle as well as any new Model S or Model X purchased before December 31, 2017. If you choose to sell your current Tesla, free Supercharging will transfer to the next owner. In addition, up to five friends you refer will also receive free Supercharging."
So, place your order before year's end if free, unlimited Supercharging is something you desire. Or perhaps wait for Tesla to again change its Supercharging policies.
Source: Teslarati