OK, I'm not that old -- and even I remember smoking sections on CO planes. In fact, I flew IAH-MEX in 1995 on an MD-80 that had a smoking section (technically it was an international segment, though it was operated by standard domestic equipment). The plane wasn't very full, so I remember several people from the front of the Y cabin went back after take-off to light-up.

I later flew MEX-PVR on Mexicana, so my return to the States was on a then-COLite single-class configured 737 -- No smoking section that I can recall.

In late 1997, I flew CO LGW-EWR (had flown BA over) and it was smoke-free. So, the smoke-free international policy must have occured sometime in between -- just to give a sense of the timing.