I suppose the proper answer is that that very much depends on your philosophy of desktop environments; I happen to prefer modular desktops built out of separate programs from separate authors which may or may not even know about each other. This has the advantage of making it trivial to retrofit new features without needing to ask your DE upstream. Or to be more pithy: Okay, fine; what desktop environments implement keynav? With X11, I can make the answer be "(probably) all of them".