Why make it a technology cut-off? Pre-65, Pre-70, Pre-85 and Pre-90 have all used the "every five years" cut off, and it works fine. There was never as big a leap in development of MX bikes as the move to long travel suspension - everything else before and after has been incremental improvement. Even the rapid advances in the early 1980s (water cooling, front discs, linkage rear) didn't make the older bikes as obsolete as long travel did in the mid 70s.
That said, let's look at the main technological advances in the late 80s and through the 90s. I'll focus on the bikes that were reasonably mainstream:
1. Cartridge forks that worked. First introduced by Honda in 1986, mainstream by 1988. Unquestionable improvement.
2. Rear discs. First introduced by Kawasaki, KTM and Maico in 1986, mainstream by 1988. Unquestionable improvement.
3. USD forks. First introduced by KTM in 1983, mainstream by 1989.
Arguably a step backwards until the 43mm versions came out in 1992/3. Arguably not an improvement until 1996 when 45/46mm versions became common.
4. Closed chamber forks. First seen in 1996 RMs (as RWUs), and 97 CRs (as USDs). Didn't become mainstream until mid-00s.
Definite improvement.
5. Competitive four strokes. It could be argued that there was always one or two available (CCM, HL500, Husky510, etc), they only began to be taken seriously in the early 1990s with the Husky TC610 and Husaberg. The 1998 Yamaha YZF400 was the first mainstream 4T to be taken seriously as an MX bike.
Mainstream by the early-mid 00s.
6. Keihin PWK carby. First introduced in the late 80s (88 KXs, I believe).
Definite improvement over the round slides and Mikuni flat slides that preceded it.
7. Keihin PWK with Powerjet. IIRC, they were first introduced in 1998 on the KX250 (possibly '97 CR250?).
8. Carbs with TPS, allowing 3D ignition curves. Don't know the year, but 1998ish.
9. Aluminium frames. First introduced in 1997 on the CR250. Definite step backwards, at least until the early/mid-00s. Mainstream by the mid-00s.
10. Ergos. It's a big one, but hard to define. There's a clear progression toward having the bars further forward, the seat being flatter & thinner, and increasing the distance between the seat and pegs.
This change was most obvious in the 1996 YZs, but it was mostly obvious because the 93~95 YZs had dated ergos when they were released in 1993...
I don't think there's a year/period where you can say that ergos took a significant step toward what current model bikes have.
It is also something that would be very difficult to police.
11. 19" rear wheel. First seen on 1989 YZs, mainstream by 1991 (even if Honda and Husqvarna resisted for a long time).
Definite improvement.
I think that's the main ones - I want to talk about brakes, but it's much the same as ergos, so I won't.
Maybe Honda's HPP exhaust power valve from 1986 CR250 also deserves a mention, but that bike's forks are more significant IMHO.
Maybe also the move to undersquare 250 2T motors and the universal move to 54x54 125 motors is also relevant?
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All of that leads me to conclude that Pre-85 should actually be either Pre-86, or a "No cartridge forks, no rear disc" class.
Maybe Pre-90 should be Pre-92?
The next clear technology break would be 1996, 97 or 98 - in 1995, there were no alloy frames, TPS carbs, closed chamber forks, or YZF400s - by 1998, all of those things were offered by at least one of the Jap manufacturers.
Or you could say "sod all that" and simply make the class Pre-95?
