Forrest/Rampage was one of those scored fights that was ahead of its time. Leg kicks weren't really scored the same as head strikes. As time goes on the decision makes sense, but right around this time, Machida/Shogun was scored for Machida. No one really scored leg kicks unless they nearly crippled the opponent like Faber/Aldo.
I don't think there was ever an official judging criteria change that specifically changed kick scoring, but it was one of those things that naturally shifted as more fans were educated. Also there used to be more emphasis on whoever marched forward got favorable scoring, where now today it's secondary scoring criteria. It's wild how controversial this used to be, many even said robbery when the fight happened or Forrest received favorable treatment.