Compressionless 4mm shifter cable came in with SiS in about '85/'86.
Before that I believe shifter cable outer was the same as brake cable outer.
When I first overhauled my 1982 Holdsworth Avanti a couple of years back I replaced all the cables with the "obviously much better" modern stuff - Shimano Dura-Ace/Ultegra level.
The shifter cables started pushing through at the rear stay cable stop, looked around for ferrules then realised the stop was made for 5mm outer.
Replaced the shifter cable outer with a piece of the brake cable outer and no ferrule, problem solved.
It's been working fine for the last 1500 miles.
So my point is: some bikes do not want modern shifter cable.