Going back to the original post and the linked b/w portrait, I tried to reproduce the result with a play raw portrait
Made it monochrome and added more contrast with equalizer, tone curve and lowpass filter
XMP file here: MLC-DSC_1922_01.NEF.xmp (13.9 KB, Darktable 2.6.2)
The modules used for this specific effect are named ‘FX’