Plain English is about straightforward communication. It is not for every text. Shorter sentences are good. Active verbs are good. Fewer dependent clauses are good. Dependent clauses are words that hang on the main clause, which you could cut out and the main clause would still make sense. Sequencing information into further short sentences is good.
Rather as you might put your opponent’s argument first in a debate (so that you could finish your speech on your own conclusion, having disarmed your critics in advance), you can think about what you choose to write first. Whatever comes first in what you write needs to be what the reader needs to find first. What readers need to find in first place is the basic thing they need to understand. Everything else gets built on that — in more short, simple sentences.
Teachers know this. They start a lesson with the ‘big picture’ for the lesson, so everyone knows why they are there and what to expect. (Actually, they start with a ‘stimulus’ on the board, so that people arriving before the late arrivals still have something to think about but the lesson can still get started on time, at the same time.)
Teachers follow the big picture with the objectives. They spell out what is to be learned. They finish with a ‘plenary’, encapsulating what has been learned. Preachers follow the same two techniques. Preaching begins with the familiar and leads the congregation into the unfamiliar; preachers say what they are going to say, then say it, then say what they have said. Good writing in Plain English has these virtues of good teaching or preaching.
Accessibility is not Plain English, but in a broader sense of the term ‘accessible’, Plain English will certainly help your writer to reach their audience. Not all animals are cats but all cats are animals; not all accessibility is about plain language. Accessibility is about the machinery that goes into the document, and making that work smoothly — especially for those who use screenreaders, but also for those who may find it hard (for all sorts of reason) to process the formats which computers put down on the screen or the page.