The Whitmore Section, William Thornton and Lini, July 2011, Modern Steel Construction, discusses this code provision.
Thornton and Lini said:
Although Whitmore’s findings were published in May, 1952, widespread use of the Whitmore section did not occur until the late 1970s. In fact, the method was not widely presented to the engineering community until 1974, when it was discussed in Fisher and Struik’s Guide to Design Criteria for Bolted and Riveted Joints.
According to that same article, the Whitmore section is "in the manual" as of the 14th edition. (I haven't looked to trace it backwards as to when it first appears).
The Whitmore thesis
Whitmore, R.E. (1952), “Experimental Investigation of Stresses in Gusset Plates,” University of Tennessee Engineering Experiment Station, Bulletin No. 16, May
