They are still available from GM. Part # 15569393 list is $357.45. As I said, I peeled 1000's off over the years, the tailgate looks better smooth, in my opinion. I'd always let the customer know I'd peel it off for free, but it would be trash and to replace it would be in the $400 range with labor. Catch it on a hot day, get one corner started and away from the metal a little, grab on with pliers or vice grips and pull your heart out.
It is solid double back tape on the back side, not just little strips, and the band is thin aluminum (or some sort of metal) so there is just no way to get it off without severely distorting it.