99-00 rule change

I can't quote chapter and verse, but I believe that it had to do with how far the nose of the saddle can be/ must be behind a perpendicular line drawn through the center of the bottom bracket axle. (I could be wrong)

So should I not buy a 1999 bike for triathlon?

The rule only applies to UCI sanctioned events. The UCI is not the governing body for triathlon. In the U.S. he triathlon sanctioning body, as recognizes by the International Olympic Committee is USAT.  If you plan on riding the bike in UCI sanctioned Time Trials, and the bike does not meet the regulations, then pass on it. If you are only doing tri's it makes no difference. USAT allows all sorts of bikes that do ot meet UCI guidelines (Softride, Y-bikes, Cheetas, and others) USACycling is the governing body for cycling events in the U.S. I am not sure what their take on the geometry rules is, but usually they follow UCI, but maybe a year or two later.

Thanks