I've always thought they were metal boots to begin with. The reason they look like robotic legs is because of how the puppets were made. The puppets feet kind of look like they were made to actually be able to move resulting in their ankles to be thinner and adjustable.
Any other actual artwork of Fox showing his feet they look like boots. The ankles are much thicker and they don't really show any real signs of robotics.
Actually the metal boots from Melee they don't even look like they should be functional even as boots. But if you look at this model from Star Fox 64 3D.
http://static2.wikia.nocookie.net/__cb20130617015144/ssb/images/6/6d/Fox_(SF643D).png
You can kind of see where it looks like his boots are partially material and partially metal. This is probably what they had in mind from the start since 64's Fox and SNES's Fox are still very similar in design for the most part.
http://media1.gameinformer.com/imagefeed/featured/kyleblog/features/literalnames/foxmccloud_610.jpg
Color scheme aside, the jacket, jumpsuit, gloves, boots and headpiece are all there. He even has the nametag in the same spot on both, the belt buckles are both generally the same shape, and to go even farther with it, they both have their sleeves rolled up.
All of Fox's boots are just boots with metal parts to protect his shins and downward. Basically they're steel-toed boots with shin guards.