… you may disappointed here. Perhaps preserve the chrominance is not the right wording here. Preserve color could be more accurate.
Despite the maths are the same in filmic and in tonecurve for that option, I get different effects on my images.
Applying the norm on tone curve, beyond preserving the color, limits the increase of saturation brought by the curve transformation.
On filmic side, when I enable preserve the chrominance I get more saturated color.
I don’t know where this difference comes from.