As
TheFourthChaser said, Smogon does not enforce Item Clause in its main 6v6 Singles format of battling. VGC, the Nintendo event format, does feature Item Clause and is also a fun way to play.
That said, with XY, many different types of items are quite viable and many of the teams I make seem to adhere to item clause even without intending to do so! Items like Choice Band, Specs, Scarf, Expert Belt, Life Orb, Leftovers, Mega Evolution items, specific type boosting items like Sharp Beak and Spell Tag, Weakness Policy, type resist berries like Chople and Wacan, pinch berries like Liechi and Salac, Rocky Helmet, Assault Vest etc. are all very viable and most Pokemon are better off making use of one of those items instead of Leftovers anyway!