I'm from california and it says voluntary on mine although I'm debating on getting it. I think it'll help with my mpg as my tail pipes are pretty black and its saying 26.7mpg, which is sort of low comparing the results on this forum. I already had to get reflashed once for a recall and before that I had atleast 28mpg.
I just recently had to get my car smogged and it was different process than most times I had to get smog. Smog guy didnt plug anything in or put it on a dyno. All he did was open up a book and check something under the hood and that was it.