Apple haven't provided X11 support in OSX for some time now. The last X11.app was OSX 10.7. It's now a separate open source project as XQuartz of which Apple do contribute to but it's pretty clear they don't think it's that important. There's no support for retina displays for instance. The chances of them ever supporting iOS would be approaching zero. IME it's never been that great on OSX even when they did support it fully. It was more of a hangover from NeXT.