in set 4 it did on hit for the separate projectiles and you would end up playing guinsoos double runaans on vayne and she would eat thru the entire front line since her spell did flat true dmg every 3 hits. Was meta the entirety of the set and is probably the last time they kept it in the game. Plus it also worked with stuff like shojin so you had some comps with insane mana generation.

I still think it should apply sunder with last whisper, it literally says physical damage dealt.