I believe stickwiglr is thinking of this transition: (from an AOPA article)
"Prior to August 1997, the FAA required a high-performance endorsement in order to act as pilot in command of an aircraft with more than 200 horsepower or that had retractable gear, flaps, and a controllable propeller. Many aircraft met this definition and were used to gain the training and endorsement, including the Cessna Cutlass 172RG, Cessna 182, and Piper Seminole, to name a few.
With the changes to Part 61 in August 1997, the FAA changed the definition of high performance and added a separate endorsement for complex aircraft. The definition of high performance is now "an airplane with an engine of more than 200 horsepower." This ruled out numerous aircraft, including many Mooneys (200 hp), Piper Seminoles (180 hp each engine) and most of the Piper Senecas (200 hp per engine). This seemingly simple change had a significant effect on the training industry. Many flight schools needed to add new aircraft to provide this training."