Apple still use the old version for their hue/saturation effect within the basic category of FCP X but it is not available in the Motion library.
They work quite differently. Desaturating with the old lifts the luma while the new version mainly brings the red and blue channels into line with the green.
Annoyingly the new version shifts the luma slightly by default i.e when applied to a clip in FCP X without changing any of the controls.
The old clamps blacks at 0 on the FCP X waveform monitor.
The old version allows better tonal separation in the midtones so if you use the color balance filter and drag the green midtones slider down and the blue up you will darken grass and blow out blue skies. The new performs more of a global adjustment to luma, with green changing the signal more than red which changes more than blue. The old is arguably more useful for B&W effects as long as you make a global adjustment to bring the levels down.