It was proposed as a change to the 2020 language to remove "panelboards" and add additional wording to allow engineered designs.
The new language that is proposed for 705.12(B)(3) is:
(b)Where two sources, one a primary power source and the other another power source, are located at opposite ends of a busbar that contains loads, the sum of 125 percent of the power-source(s) output circuit current and the rating of the overcurrent device protecting the busbar shall not exceed 120 percent of the ampacity of the busbar.
(e) Connections shall be permitted on switchgear, switchboards, and panelboards in configurations other than those permitted in 705.12(B)(3)(a) through (d) where designed under engineering supervision that includes available fault-current and busbar load calculations.
In the last two versions of the code, 2014 and 2017, the 120% rule only applied to panelboards. NEC 705.12(B)(2)(3): "Busbars. One of the methods that follows shall be used to determine the ratings of busbars in panelboards."
It was implied that other than panelboards did not need to comply with the 120% rule since it specifically only called out panelboards but many people were still applying it to all distribution equipment, in 2020 it is being made more explicit. The reason for this is that panelboards are used in residential and small commercial systems that are less likely to have engineering input so the requirements were more prescriptive. Also, panelboards tend to have more thermal issues than switchboards and switchgear due to the smaller size and less airflow.
I have to admit that I thought it was a mistake in the NEC for a long time and assumed that the 120% rule was intended to apply to all distribution equipment. It was only after I started asking around that I came to the conclusion that the NEC was written correctly, if unclearly, and it was not intended to apply to anything other than panelboards. The 2020 language clears this up.