That tradition, in fact continues into this day, though it is becoming rarer, with professionals training with the team year-round, instead of being left to their own devices in the off-season.
One of my friends recently came into possession of such an off-season fixed-gear frame. It's a De Rosa, with obvious road geometry and brake drillings, but track ends and 120mm rear spacing. It is painted in the purplish blue and turquoise of the old Batik team De Rosa used to sponsor in the late '90s. It was obviously never meant to see a velodrome, but it is a highly impressive piece nontheless. (It's all fillet-brazed as well, which I understand is incredibly rare for De Rosa who, by 1997 had already moved to TIG welding for their steel joining.)