If we're talking mainstream Hip Hop, it never really left. Look at the Hypnotize Minds releases from the 90's. Those were fully produced by DJ Paul & Juicy J. Cash Money's releases were fully produced by Mannie Fresh. There were also producer/rapper combos like Missy and Timbaland who continued to work exclusively with each other up until 2003. There's also OutKast who either had Organized Noize lacing their albums or Mr. DJ and themselves producing their albums. There was also The Clipse and The Neptunes.