Correction, acid corrodes steel. Other venom, not so much
That is the thing about Poison type- because it includes venoms, toxins etc. AND acids- they could have gone either way with it, but they decided to go the toxin route by having it ineffective against things that have metallic properties which does make sense- though when you use 'Acid' on like Registeel, you;d expect that b*tch to start to corrode! Though that's the way they decided to play it. I don't expect them to change that now, it would just be strange for something to be immune then suddenly extremely weak to something.
Then again, I've always held out for some kind of Poison type attack or status move that made Steel types weak to it for a period of time or something. Maybe like an Acid Rain weather condition that causes Steel, Rock and Ground to become weak to Poison for the duration. It would be like an anti-sandstorm considering how they've buffed that to help their special defense or whatever.
Maybe, just maybe, there could be a legendary that summons that weather condition! That would be awesome! And it would turn Poison into a fearsome attacking type for part of a battle, which would be cool
Ooh ooh, I'd like a Druddigon evo that is more like a gargoyle (France, Notre Dame etc.) that is Dragon/Rock- that would be awesome!
ANd I was TOTALLY right about the romanized name ;D of the region!