It started as a joke for my friend who doesn’t understand how I use a 35% but then some people seemed interested so I actually made it.
I’ve been using it for about a month now and use the QAZ keys for macros. I have tried actually using them as Q, A, and Z, too and it was actually easier than I expected.
If you leave off the amusing extra keys on the side of this one, the idea of "40%" boards (actual percentage of 104 keys varies) is to minimize finger movement while touch typing and to move lesser-used keys onto one or more Fn layers like laptops (or really most keyboards these days) have.
I don't really touch type, so while I still like weird stuff (and make it myself), the 40s scene is a bit lost on me, though my very first hand-wired keyboard build was a "Planck" (somewhat similar to this but with the keys in a perfect grid) with three extra keys.
Even for people who do touch type, there's a definite learning curve when you move to a 40% or lower board. I found that the muscle memory starts to build up pretty quickly so after a bit of stumbling around I got used to smaller keyboards without too much trouble. The main problem now is when I try to write code on anything larger than a 40% I make constant typos.