I’ll point… as I often do… to Spire: The City Must Fall by Rowan,Rook, & Decard.
The core rules are described early in the book. They use clear and concise language. Little attempt to be anything other than instructional.
Later in the book, when they describe the world of the game, they make much more of an attempt to evoke the feel of the setting. I wouldn’t say that they’re worried about the quality or cleverness of their prose so much as they are at inspiring readers to think of interesting things to use in play.
With the character classes… which are all very specific to the setting… they use both technical writing and creative writing, with the creative flavor text appearing in italics and the rules in standard text.
I appreciate them separating things in this way.
And although I find their world very interesting and, it’s far less about the presentation than it is the content. They have great ideas… they’re not trying to be clever about how those ideas are shared.
It’s far more important for a rules system to be clear and concise, I’d say.