I commuted on the GN for 25 years, covering both the WAGN and FCC periods. This is not my recollection.
There was definitely a softening of driving during the late WAGN / early FCC days, but not quite as dramatic as suggested.
In olden days trains would likely still have been doing upwards of 75 mph by the time they encountered the single yellow at Hitchin South, which would be followed by a hefty brake application bringing the train down to 40-50 mph by bridge 99A (the footbridge), which was about the point the junction signal would clear if it was going to. It was then a case of coast through Hitchin and take the turnout (I guess a few drivers might have reaccelerated and braked again, but this would be the minority), or slam on the brakes to stop at K699.
What changed was defensive driving which would mandate a significant reduction in speed immediately upon encountering the double yellow.
Likewise, there was also a change for trains taking the 70mph DF to DS crossover at Hitchin South. In old days trains would hold their speed as long as possible (potentially 100mph), slam on the brakes on the approach to the points, pass over the crossover at 70 mph, and still have plenty of room to brake for Hitchin station. You don’t tend to see that now, though it does very occasionally happen. Now it’s much more common to pass over the crossover at about 40-50 mph. Again, defensive driving because the junction signal will have been showing a single yellow. This crossover is quite rarely used nowadays.