Ever been cross country skiing on snow that feels very springy?
Some snow types are both elastic and of low enough modulus to get significant deformation according to the stuff I have read.
I doubt you will find this kind of snow in a properly prepped race course but on a groomed hill with very hard packed powder I think the snow can be an important factor in the energetics of the turn.
I'm not sure I would use the term "Plastic deformation" in regards to snow (not slush), what I see is a kind of brittle fracture followed by repacking to a density that will withstand the compressional load of the ski.
With slush you get into the realm of fluid mechanics more than fracture mechanics.