Why do I think every programmer should try Plan 9, use it for a while, and strive to understand the design decisions which went into it?
It's not because I think it'll become your daily driver. Humanity has almost certainly missed the boat for Plan 9. No, I think you should use Plan 9 because the ideas are important and they will frame your thinking for the future. The simple fact is that programmers who understand Plan 9 are better programmers than those who do not.
In being different from mainstream operating systems in important and subtle ways, Plan 9 draws your attention to these things. It causes you to question the things you take for granted in your operating system. Modern Unix systems are so samey that, even to programmers who have used several - Linux, BSD, macOS, Haiku, etc - many of their ideas can seem invariant. In truth they are not, they are design decisions that were once made and they could have been made differently. Maybe some of them ought to have been.
Plan 9 is a light under which the truth of the operating system you're using today becomes apparent.
This looks like a cool project.
"The Open Book aims to be a simple device that anyone with a soldering iron can build for themselves. The Open Book should be comprehensible: the reader should be able to look at it and understand, at least in broad strokes, how it works. It should be extensible, so that a reader with different needs can write code and add accessories that make the book work for them."