I don’t think Monsters have Poison Duration Reduction on any Difficulty. At least that’s what I remember seeing on Arreat Summit years ago. All I remember was Drain/Chill Effectiveness, Resistance/Immunity, Block Chance(ugh…), Monster HP, Attack/Defense Rating and what Base Damage they do. So unless it actually exists somewhere online(where someone data mined it), I haven’t seen it and therefore from a PvM perspective, it isn’t needed.
The only reason Lower Resist might enhance or increase Poison length is for PvP given that Fade and now Plague runeword now exists… It’d be a way to counter the length reduction but Cleansing/Fade also reduces Curse duration too(which there is no work-around on that front unless Curse duration was substantially increased…)