In terms of reducing intensity, there is no difference between adding an aperture to the lens (in front of it or behind it) or cutting out pieces of the lens. In a sense a cut lens is just an ordinary lens with a different type of aperture. In any case, the intensity is proportional to the area of the aperture/remaining lens.
As an interesting side note, the focused parts of the image would stay the same shape as before cutting. Defocused parts would have blur spots with the shape of the remaining lens, like in these two images (taken from here: https://en.m.wikipedia.org/wiki/Bokeh), instead of circular as with a whole lens (assuming the lens is circular, of course):
Maybe this helps visualise what’s happening here: you’re cutting off pieces of the image each object point makes. When the image is sharp, you don’t see the shape because its size reduces to a point. But the shape is still there and you’ve still cut off pieces of it, removing those pieces’ contribution to intensity.
