This is not a complete list by any means; it’s just my attempt at not completely wasting my PCS membership. So most of this will be articles in PCS’s journal. More will be added later when I encounter them.
Cheung, I. W.
Plain language to minimize cognitive load: A social justice perspective.
IEEE Transactions on Professional Communication, 90, 4, 448–457.
References social justice and human rights aspects using an explanation based on cognitive science. Although the journal calls this a “tutorial,” I don’t think I’ve seen plain language “rules” ever explained like this before.
Schriver, K. A.
Plain language in the US gains momentum: 1940–2015.
IEEE Transactions on Professional Communication, 90, 4, 343–383.
Contains references to findings about the inaccuracy of readability metrics.