421c - Right, except you won't find the unfeathered numbers on a Conquest I, which was my introduction to the AF. When I asked, and asked, turns out the unpublished numbers are so horrible that the AF is required, so much so, the manufacturer took feathering the prop completely out of the hands of the pilot.
Whether the AF is optional on the King Air or P180 is certainly possible if it was installed as a pilot helper, a safety enhancer because on those aircraft an unfeathered prop is not the end of the world.
What AF really does is immediately feather the prop, and the right one, where pilots take some time to figure it out, as feathering the wrong one has killed people, and since I teach this stuff, it's one of those, 'better get this one right type of things'. So if VMCA is higher on say a Piaggio, with out the AF, that tells me they are trying to keep the pilots at a safe speed unfeathered, until they can get the prop feathered. In the Conquest there is no unfeathered VMCA, where I assume it doesn't exist, it's that bad.
The point being is that if the xlist says to check the AF, yeah you better check it, or your are playing test pilot.