Diversity became a buzzword amongst the brass during Clinton’s regime, circa 1997. I don’t know it’s more primitive sources, but at that time it was being used to mold upper middle management selection and manpower criterion.
It started well before then. It was well in process by 1984-6. My own thesis is that it was picked up from Academia by 0-5/0-6 grade officers serving as Professors of Naval/Military Science at ROTC Units who then rotated back to military graduate schools like Command & General Staff College and the Pentagon and brought the diversity meme back with them like a STD.