I think I recall several companies producing system free supplements back in the late 1990s/early 2000s. I know of at least one system free supplement that did so poorly, it stopped an entire line of splat*books that were to follow it. 

I like the idea of system free books, I just don’t know how feasible they are from an economics standpoint. Also, does it matter who the target audience is? Would it work as OSR material? What about Cortex or even FATE?