The rule is applied to the chassis conception, so with my RC10 even though itself is a bit later than 1990, the chassis design is 1990. As Stu says, for 2wd defo no XX's or B2's, anything other than that is handicap enough I think.
4wd alledgedly the procat was old enough
G's car was proper good, would have put many a more modern 4wd to shame indoors!