The presets system got revamped and with it the way to edit them also changed. When it did some of the old presets started to look a bit different (Not bad, just with certain proportions or colors offplaced); with it, it also removed some of the old preset system options like the one you mentioned. The Random Body Generator was necesary because the way the spawning works was like this:
If RBG (Random Body Generator, for short, lul) was on, when spawning a neph, the game would pulll a preset, then apply a random body trait modifier, such as chubby, slender, etc
If RBG was off however, the game will always pull the presets "exactly" how they look by default and thats the appearence they would have. Even if they come with the chubby trait by they're the counterpart of endermen, they would still look thin cause the preset made them look thin by default.
With this new system each preset has a subsection when those proportions can be applied on each preset individually. On the previous system, the slender/chubby/whatever configs relied on general proportions so if you had for example flat chested girls, the moment u hit RBG, u had to say goodbye to the flat chest cause any of the typical shapes would assume the girls have some boobs. This time however, the configurations of each of those body proportions are present independant. So the preset saves the default appearence of the neph, but also contains the numbers on how a slender variant would have, same with chubby, etc.
Basically because the body proportions are now present independant, there's no way to aply a general body proportions modifier so that menu option basically turned both obsolete and useless with the new system implemented. For better or worse ^^'
