Is it illegal for me to not pay a fee after I left that university?
You don;t explain what the fee is for, so its difficult to answer. If the fee is for something you knew required a fee at the time you received whatever the fee was for, then you do owe the money. If the fee is legitimate, and the school informed you prior to obtaing somethng that there was a fee, you are obliged to pay it. It may not be illegal, but it certainly would be immoral for you to claim exemption simply because you forgot to pay it while you were a student there.