Eyes off my screen: Apple patent outlines display tech with adjustable viewing angles12
There are, in fact, “privacy screen protectors”, which severely limit visibility and only make the display's information observable if you are looking at the screen dead on. That sounds nifty, but there's the caveat that it's a physical screen protector — it's either on or off, and you need a new one if you want to put it on again.
It seems that Apple is looking into making the “privacy display” a toggle-able feature on future iPhones and iPads. The company has filed a patent (dated February 2016) that explains ways to adjust an angle-of-view feature at will.
The patent describes a display that “may” have a backlight unit, which has a “light guide” embedded in it. Said guide works in tandem with a lens array, so that the light rays and lenses can project an image in a certain direction only.
We have to imagine that this plan would have to be drastically re-drafted if Apple goes ahead and makes its displays AMOLED. Rumors are insisting that the Anniversary iPhone would have such a panel, so it's probably a matter of time before it reaches iPads and “regular” iPhones. Who knows, the tech might actually be applied to Macs only. And yeah, a patent filed definitely doesn't mean a feature is actually coming.