I'm taking a look at the chapter on sprites from a NES programming guide at famicom.party. There is a little table which describes what the different sprite attribute flags do:
Bit # Purpose 7 Flips sprite vertically (if "1") 6 Flips sprite horizontally (if "1") 5 Sprite priority (behind background if "1") 4–2 Not used 1–0 Palette for sprite
I'm generally amazed by how well the memory space is utilised in the console, so I was surprised to see that there are 3 bits per sprite which are not used. Two of these bits could specify a rotation orientation of the sprite. Why was this not implemented on the hardware?
Were these bits used by game programmers for their own sprite-specific information, potentially unrelated to graphics? Were they left unused intentionally for this reason?