This is a good example. Yes, shadows on the wall. And it is not well defined what is a wall. And these walls are not flat. And to analyze their shape, we use the shadows.The shadows on the wall seen by Plato's prisoners are real shadows. Because they follow from the really existing objects, the really existing source of light, and the really existing wall. Naming them illusion explains nothing. An explanation has to describe how the illusion emerges, in a logically consistent way.
BTW, returning to the 'observables' which are not obserables at all: can you see elementary particles with your naked eye? You do you use some devices to get values of these 'observables'?