As far as I know, it's not available to order on any trim level right now. If it is/was then it would be available on Overland trims. Unfortunately, Summit and Summit Reserve don't get the off-road package as option.
I think they figure people won't want to offroad their 70K vehicles. However, the Mercedes G550 has a starting price of 132K. While the Wagoneer and Grand Wagoneer are a better comparison to the G550, I feel they have missed a segment in the middle here which they could easily make a few extra bucks. Personally, I want the offroad package plus everything the Summit Reserve has -- except the ridiculously large 21" wheels.
I personally plan to purchase some 18" wheels for when I need them, and am hoping that Jeep was kind in designing the assembly such that we can simply just order and bolt on the the skid-plates ourselves without having to drill, cut, or weld anything.