For me the game changer was suspension. Until about 93 (?) the design focus was on top class steel/ti rigid frames at the top of the range. From then on the focus was on suspension which means design at the top end moved away from steel by and large. So if we're thinking about the classic idea of a retro mtb being steel (not everyone's cup of tea I know), for top end bikes, post 94(?) is more about the start of the suspension take-over (as it should be in terms of evolution of the sport) than the refinement of rigid steel. Having said all that I wasn't involved at the time - this is just my observations from being involved in recent years. Happy to be put right by Chopper or anyone else!