I've seen plastic camshafts or plastic lobes, on older Briggs engines (but not vintage ones) such as the Intek series.
I've seen plastic timing gears and plastic cam lobes but never a plastic shaft. The steel shaft on the gear end goes all the way through and the plastic part is hollow, right?
It'll probably last a lot longer than you think if they haven't changed the material composition. The flathead governors, the gear portion, are also made of that milky white plastic and they last and last but then again it is under comparatively less stress than the camshaft.
Weird stuff.