No reasoning, it’s just not implemented. There is a comment in the code saying it should maybe be exposed to user interface. Also, it’s enabled without telling you if you enable softproofing.
There is some gamut “safety-jacket” in filmic, but then exports are handled through Little CMS2, which does the perceptual thing to gamut map. So, no manual compression.
Well, sorry, but at some point you need to start at the beginning… What is an histogram for ? What are you trying to see in there ? Once these questions are answered, the choice of space is pretty much done. The point is not to be condescending but just to remind users to disregard scopes they don’t understand because they don’t need them. Retouch the picture, and maybe if you see something wrong in the picture, then scopes can help you troubleshoot the issue and spot the origin of the misbehave. But as long as picture looks good, don’t make up problems.
The only use for an histogram is to see how the image fits in the available range, between medium black and medium white. So you only care about how much space you get on the left and on the right, if there is any space at all, or look for a peak at min/max intensities that would suggest clipping.
Any space will do that. Now, sRGB, because of the gamma/power 2.4 will put middle-grey in the middle of the histogram, if that’s what you want, but will not have an uniform spread because it has actuall 2 gammas (one for midtones + highlights, one for dark tones). The PQ curve from Rec 2020 PQ dilates shadows a lot and compresses highlights, so it gives you better legibility near black and worse near whites. But it’s still the same span, only zoomed differently. The HLG curve is more balanced.
Whatever space you choose doesn’t change the meaning of the histogram span : you have some range between white and black, you want to see how the pictures fits in that range, period. The shape of the histogram will change with the OETF of the space you choose, but the shape holds no meaning at all, so it does not matter.