You can select the skins in the Collections menu but they don’t work on Classic mode.
It would still be nice to see thrall, arthas and jaina as heroes in versus games on Classic graphics and they are still advertised as " Warcraft® III: Reforged: Multiplayer hero skins"
However due to playability advantages with Classic graphics they will never see the light of day again in MP. It also doesn’t say anywhere that these skins are exclusive to the reforged graphics when buying…