I disagree. Watch any of the curb god videos and literally everyone lifts for slappy blunts, overcooks, and feebles. Try to tell Robbie Rodriguez his slappy form is wrong when he's likely got slappies on lock better than almost anyone on here.
A slappy back NG is possible if you still slap the front truck up first. It can be cheated by doing it super slow and see-sawing onto the curb sure. I'm thinking of carving super hard like a BS 50-50 and right after the front truck climbs you lean forward. The back truck would still hit and if you hang your back toes off more than normal and lean forward you get right into it. Same with FS. John Rob Moore has posted a bunch of them recently on IG stories. Myself and friends have definitely done all of the variations using this general form. You do hit the back truck and maybe for a split second it's grinding, but if you time it right the back wheels climb but the hanger never touches. It's scary and kinda violent.
I'm also of the opinion that people that get super anal about slappies are kinda missing the point of what they're supposed to be in the first place. There's a few that I think you should definitely never lift for. Back in the day we did them at the end of the day when our legs were shot while switching to party mode it was more about shooting the shit with the homies rather than treating it like Olympic figure skating.