Very good question!
The MBTI Manual (third Edition, 1998) briefly uses them, p30-1, in the form of subscripted small caps (ES[SUB]I[/SUB]T[SUB]E[/SUB]J, when establishing the basic type stack, and afterward, in the following chapter on the types. Doesn't seem to be in Gifts Differing and Was That Really Me, which write the words completely out. So it's probably the manual, in which it was clear that the functions are seen in the "natural" form, and the attitudes are simply footnotes, indicating orientation.
So it was likely later writers who took that use from the manual, and popularized them (and not sure whether that edition was the first or not). Not sure if Berens was the first. May have been others before. I know when I first entered the online type community, on Yahoo, before joining these boards, the shorthand was used, and yet it was a place heavily influenced by Berens (she had even been there), but I don't know if she's the one who popularized them. They seem more widespread than that.