It was not restricted. It was used several times but the thing is a sharingan user needs to be in a certain level of skill/strength/knowledge to copy another person. Also sharingan's main ability is predictive eyesight which allows a person to see execution of a technique (if something moved 10× slower you could understand it better) more clear but copying aspect is more about being a fast learning genius (most main sharingan users are geniuses -Kakashi, Sasuke, Itachi, Madara- that't why people think copy is the direct result of sharingan while it is not the case for all sharingan users).
For ex. Itachi is a suiton user so he can theoriticly copy every other suiton with handsigns; but we all know he can not copy Kisame's great water shark bullet because he does not have reserves for that technique. Or he (or other sharingan users) can not perform FTG by watching Minato teleporting because the tech has some internal dynamics and knowledge which can only be learnt by knowing.