: You're dead wrong about CE, though. She's got natural-looking shading across
: normal human skin contours everywhere. See her exposed collarbone in your
: images that's darkened in the depression,

This is why I said it's less detailed. In the originals, the geometry detail is much less, and even the textures - the lines and dark tech-regions - are less detail. They couldn't show any kind of bodysuit 'lip', but those dark regions imply some form of bodysuit; as I mentioned the collar line (even a relatively skin tight body suit can have a defined collar) contrasted with the 'open' collar of Halo 4. All these things together make Halo 4 MUCH more arguably naked-style than the various factors of the previous designs which imply bodysuit.
She not 'naked' in Halo 4 in the sense that her 'private lady parts' are discernible, but the composition of her geometry doesn't imply bodysuit - at least not nearly as much as in the previous games, Anniversary included.

If we're thinking lore-centric, then up until Halo 2 she had a very clear body suit (as depicted in the Anniversary cinematics); in Halo 3 I would argue she still had a semblance of body suit due to the darkened collar line (and lesser detail which one could alternatively argue was limited by tech; but with which I'd disagree ;P). Suffice to say, for whatever reason, Halo 4's visual esthetic for Cortana's design certainly differs from previous games.

Anyway, my original point - Cortana's "naked" factor can really only be argued for Halo 4 at best, and maybe debated with Halo 3. Before then, it was a clear bodysuit.

: and even has a belly button.

More like a concave belly, which can be expected with most any bodysuit. A 'hole' at the belly button would give a much better argument for an exposed 'skin' region (I say that to distinguish full 'naked' from attire that covers certain regions while exposing skin in most others, like cloth strips, eg)
Which, on that point, raises the question of what she may be 'wearing' in Halo 4. But I never said she was naked in Halo 4, only that her depiction is much more blatantly implying that direction. Halo 4 Cortana could still be accurately produced as a relatively safe PG cosplay :)

Remember, this is based on revenant's comment: "Remember, Cortana is naked."
I say nope. The closest example you can get to that being true could be Halo 4, depending on how you argue it.