Plain language is not one thing that can break — it is three. A reader must be able to find what they need, understand what they find, and use it. Apply repair moves to a broken document and watch which gauge each one actually moves, then hand the draft to a reader and see whether they can do the task.
1 · Pick a document
2 · Apply repairs
The tag on each button is the point. Writers who assume every complaint is a vocabulary problem simplify words that were never the obstacle. A find failure is immune to a better definition, and a use failure survives a sentence the reader understood perfectly.