1) Message Density - there's a great quote attributed to Mark Twain that nails this "If I'd had more time I would have written you a shorter letter".
In practice Jeff Bezo's push to making executives build "4 page memos" for meetings outlines this idea clearly: "the reason writing a good 4 page memo is harder than "writing" a 20 page powerpoint is because the narrative structure of a good memo forces better thought and better understanding of what's more important than what, and how things are related"(1)
2) Message Accessibility - Making your message broadly accessible is a good forcing function to writing clearly. An easy "hack" in this department is to utilize the "gunning fog index"(2) which is a structured way to estimate "the years of formal education a person needs to understand the text on the first reading" and there are many tools online that can spit out a score.
(1) https://slab.com/blog/jeff-bezos-writing-management-strategy...