Both 발이 밟혔어요 and 발을 밟혔어요 are correct. (제) 발이 밟혔어요 means "My foot was stepped" (저는) 발을 밟혔어요 means "I had my foot stepped/Somebody stepped on my foot." Another example, 나는 그에게 손을 잡혔다. He grabbed me by the hand.