Since it is a product that was available for sale, even if it isn't currently, it still seems like stealing. It doesn't seem different than accessing DLC without paying for it, especially if it is the type of DLC that is already contained in the game and you essentially buy a download code.
Legally it might be a gray area, but morally, I don't see a difference between that and downloading ROMs. Either way, you're acquiring a copyrighted product.