tothepoint;
We purchase a number of gaskets for out Plant components and I have never seen a gasket material fabrication standard. Our gasket suppliers will give you material and maximum hardness values for the material, but nothing beyond this information. I would suspect each gasket supplier may have their own specfic requirements related to gasket composition and hardness based on experience.
We rely on several gasket suppliers to provide the required gasket material for the intended service conditions. I do know that one supplier that we have used no longer provides "soft iron" gasket material because it is not commerically available. Instead they supply low carbon steel, which has a hardness close to "soft iron" and has worked well for us.