Raking back through the memory, most of the OE sprockets I've handled, mainly BSA and Rudge but also Norton, Suzuki, Honda and Ducati, have been through-hardened - certainly tough enough stuff to resist the drilling of "lightening" holes, to which I was addicted in my yoof. That sprockets be hardened (and hard) is the SAE recommendation. (A distant memory says that manganese steels, which work harden, are particularly suitable.) I imagine the requirement is that they be hard enough to resist wear on the teeth, but not so hard that the sprocket spline wears out the sleeve gear (G12) spline.
The only Vincent sprocket I've replaced, which I'm sure was original, was definitely heat-treated: it was worn on the teeth, but not worn out, 43,000 miles, and I thought I might clean it up with a file to get a good look at the tooth profile. Fond hope: it would have needed a grinding wheel.
