Leg or foot???
足 is used for both leg and foot, yes.
You can think of the foot attached to the leg as one thing.
Shouldn't the です be optional here?
I agree, it should.
In casual language, yes. However, in polite Japanese, which seems to be Duolingo's focus, every sentence that doesn't end in some form of -ます should end in です.
It seems around the point you finish the tree but it isn't golden is when casual speech gets mixed in with formal speech.