The softproofing button and relation to histogram and how it works seem to be a bit of a running sore. It would be good to get to the bottom of this. I was just checking with dt 4.0.0 (& Ubuntu) and the display IS changing with a change of softproof profile. Here is sRGB vs. PQ rec2020 RGB. The display changes, the histogram changes, and if you use the gamut check button, that is different also. So to me it seems to be working. Please can others check with their favourite playraw! (But remember if you use filmic6 (I haven’t below) the gamut mapping will probably reduce or eliminate any changes you might see)
I tested it a few post above using a printer profile and it also works for me in fedora and windows.
Sorry this was meant as I see no discernable difference for the testing trying to find the JPG output issue, not to suggest the softproofing wasnt working.
Steve