Poster boy for haste makes waste I am. I had not even heard of the 'NEC style manual' previous to the last couple of years if that is what it is actually called, and if I got the name right, I would be surprised at that. Something to pursue in my spare time, who has it? LOL. I'm going to make time eventually.
I believe I should have stated 250.24 (A) (5) load side grounding connection, First, instead of referring to 'separately derived'.
I can't even remember when I became aware of the 'NEC HANDBOOK' version… Which has 'commentary printed/highlighted blue'.
For instance, the 2017 handbook commentary of the above refers you to 250.142 (B)
As far as the OP's original question… "When did the NEC prohibit neutral-to-case connections on the load side."…
I'm going to see if I still have a copy of the 96 code, maybe even 93 if I did the math right. It was around that time that I started trying to be an electrician instead of just calling myself one. Shocking, I know.
Even then, in my haste and zeal I could not make the distinction between 'grounded conductor' and grounding conductor… Meant the same in my mind.
Not even sure if it was always referred to as the grounded conductor… something else to look up.