Sivir had a couple really strong periods. A looooong time ago her w was a toggle, so during a teamfight it worked on all autos. During the aoe craze she was really popular. Eventually they nerfed her Q so that it hit like a wet noodle and her early game was too weak to get strong.
Sivir was also popular after the rework when Q became physical damage, and W was an onhit. Made her laning a lot better. They ended up nerfing her ult by like 5 seconds to rebalance her, and she lost popularity.