no AF to 16x AF on Skyrim for even low end GPU's costs at most 1 FPS. It is so trivial today that there is no reason not to have it as for the cost it adds a lot to the IQ of the scene. Not sure why devs have struggled with it for so many releases so far this gen.
Since almost every dev on every console seems to not be implementing it at high-quality settings, I'd imagine it's more likely you're mistaken about the idea of it having no performance cost, considering the modicum of effort it would take to enable.