Well, "optionally" in the sense that you can non-trivially excise and shim your way into a working system (the patches for 3.14 on Slackware, at least, are non-trivial and I'd imagine it's even more invasive on OpenBSD which doesn't have, for instance, /proc or /sys). Personally, I can't stand GNOME 3.X so I haven't looked too closely, but even just a couple of versions ago OpenBSD could only do "fallback".