Im not old enough to really remember this stuff, but it seems to me that Ive read of some racers who used them and used ....but I certainly dont think this was widespread at all.
Bar-cons in the late '60's and through the mid-70's were common enough on road race bikes both here in the States and in Europe.
Almost universal on Cyclo Cross bikes of the era lots of us who were riding a Criterium heavy calendar preferred them in the tight quarters of a Crit for having hands-on-bars shifting.
Indexed integrated brake-shifters made DT & Barcon both obsolete for racing just as indexed DT did for friction shifting.
Shooting a line of far-fetched BS to non-racers has-been/still-is part of Bike Racer Culture, it's best done with a straight face, like this:
some racers who used them and used their knee to whack shift in sprints to get the jump on other riders, or maybe someone told me this once...
-Bandera