Yes, the acid damage type is supported in the game, there are a few spells that deal acid damage, very rare though. But the problem is that there is no native method of introducing acid weapons from the SRD that I know of, hence there being no use for it in game. Edit: Interestingly enough, I took a look at partsys0 with V2Brute and there are fully featured (quite good looking as well!) particle effects for acid and bane hitting something, suggesting these were originally planned to be usable (possibly through crafting) in vanilla. Also the world builder can assign acid and bane properties to weapons, so it looks like half the work is done, all we would need is someone to make use of them and add ways in game to get weapons that use these properties.