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;

• 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?