+1 for this feature request
For me also looks more like a bug, especially since the behaviour of this slider is now (in v1.5.1 on Windows) different to the behaviour shown in the tutorial video
(look at 1:35). In the video when the shadow slider ist increased it affects the lowest 20% of the histogram ... maybe not perfect, but good to work with. But in my 1.5.1 demo version it only affects the lowest 10% (or less), basically making all dark tones a uniform dark grey when set to 100%. This is certainly not what I expect and is different from what similar sliders in other applications (i.e. Adobe Raw) do.
Let me also note that the highlights slider just below shows quite the opposite extreme: It affects roughly the upper 60% of the histogram, thus also the mid tones. This is something that's acceptable for me, however.
Ideally I would expect both sliders to have a comparable (but opposite) effect on the histogram, such as compressing the darkest resp. brightest 50% (or so) of the tone curve while not affecting the opposite end of it.