"You could easily say pre 90 with the same criteria then."
Yes you could, provided the bike had: no linkage, drum brakes and no water cooling
stock.
However:
1. After '82 there were virtually no MX bikes made that meet that criteria.
2. Fitting parts, eg forks, from a later model bike, that didn't meet the criteria doesn't seem in the spirit of the class.
Once you can fit late model parts, you open up the "Frankenstein bike" can of worms.
If you include a sensible cutoff date, a question that would be self evident would be:
"Could you have made and raced that bike in 1982 (for eg)?" (Not if it has '90 model forks on it.)
I don't have an Evo bike (yet), but the class appeals on several levels. One being that many manufacturers produced models that can be reasonably competitive (under the current interpretation.) So you can satisfy the nostalgia for a certain brand, perhaps from your youth.
It's also an eara that appeals to some of us "wrong side of 50yo guys" that rode these bikes in the day, and realistically only have another 20 or 30 years of riding left in us
A strictly year based class like pre '78 favours any manufacturer that made a great gain in 1977, and disadvantages any manufacturer that hit the sweet spot in 1978. (Great advances each year from '73-'83.)