Artillery has no requirement if I recall correctly, though missing out on Cannons is a drawback. For tanks you only need Oil. So modern era warfare is survivable without iron but harder without oil. The middle ages will be harder though without iron, maces is both copper and iron so there's a lifesaver if copper is present and not iron. Though attacking rifles with catapults and trebs won't be a fun adventure so the hardest loss of missing out on iron would be the cannons.