Originally Posted by T-Mar

Yes, the Superplate version of the 1st generation Deore XT rear derailleur shifted very well. I have one, with original box and instructions. The Deore XT Superplate and SunTour's MounTech were both in direct response to the success that Huret had been having with their Duopar, the acknowledged champion of wide range derailleurs since it's introduction in the autumn of 1975. Both Shimano and SunTour circumvented Huret's patent by spring loading the 2nd parallelogram. SunTour's design had poor jockey pulley seals resulting in a plethora of failures. I'm not aware of the Superplate having the same issues, though it's reputation may have suffered by association.

Frank Berto attributed it's demise to added cost from it's complexity and maintains it was dropped in favour of the simpler, 2nd generation, slant parallelogram design, after SunTour's patent expired. However, 2nd generation Deore XT didn't arrive until the 1987 model year and Shimano had introduced slant parallelogram rear derailleurs two years earlier on New Dura-Ace. Then again, Deore XT also had to wait two years for indexing. Maybe it was a case of not wanting to introduce major changes too soon, combined with paying off the old tooling.