Pixel art as used today isn't quite the same thing as old-school icon design. Unciv doesn't need to operate with a restricted color palette and even on mobile has quite a few pixels to work with, but has to make UI elements easily recognizable and distinguishable at small physical sizes. I think the design constraints are probably most similar to early iOS and some early OS X icons, rather than the constraints of Win95 and 2d video game consoles (where all art—icons and otherwise—had very limited color and pixel counts).