Is this actually a thing about French bikes?
...I think the stems stayed at more or less the same angle. IN the French bicycles I own/have owned from the 1970's, the head tube angles varied wildly over the years. A lot of them got pretty steep when that was the thing. My reference points are Peugeot PX-10's and Moto Grand Record's. I've had examples of each that were both slack and steep from early to mid 70's... I've never noticed a difference in stem angles.