Your D800 can be damaged by trying to mount a non-AI lens on it. It's not actually directly a sensor, it's a tab that engages with the AI coupling ridge on the base of AI and AIS lenses and later, which connects them with the meter. If you look at the front of your D800 without either the body cap or a lens mounted, at about the 1 o'clock position you will notice a small black plastic tab protruding--it's above and to the left of the lens mounting index dot.
That is what can get damaged if you try to forcibly mount a non-AI lens. There is no cutout on the base of a non-AI lens for that tab to fit into, and so the rear of the aperture ring on the lens can bend it or push it backwards, neither of which are good for it. AI lenses, or AI converted lenses, have a bit of the aperture ring cut away to accommodate that tab, and its range of travel as the aperture is changed by turning the ring. (At one end of the cutout, there is a small ridge that engages with and pushes on that tab, letting the camera's meter know the maximum aperture of the lens, and where the aperture has been set.)
Cameras which shot non-AI lenses natively did not have that small protruding tab in the lens mount, they had a pin that engaged with the "rabbit ears" on the pre-AI/AI/AIS lens aperture ring. Cameras which came along during and shortly after the transition from pre-AI to the AI system had a way to move that aperture tab on the camera body out of the way by pushing a button on the camera's lens mount and either flipping the tab up out of the way (F3, F4, FE/FM--early ones, I don't think the FE2 for example has this feature, the Nikkormat FT3, and some others) or by lifting the tab out of the way as it's located at the end of a lever (F2A, F2AS). And while that allowed those cameras to mount non-AI lenses safely, it meant that those cameras could only meter in stop-down mode with those lenses.
What that meant was that the meter reading would not automatically change as the aperture ring was moved--there was no connection, with the aperture tab on the camera moved out of the way to mount the lens, between the aperture ring on the lens and the camera's meter. So what you had to do was to set the aperture on your non-AI lens, push the stop-down (depth of field preview) lever/button on your camera to close the aperture to what you had set it at, read the meter there, and then input the correct shutter speed to match your chosen aperture before taking the picture. So it worked, but it was quite awkward and slow--which is why most people had their non-AI lenses converted to AI, as they could take advantage of rapid wide-open metering and focusing.
In short, don't try to attach a non-AI/unconverted lens to your D800. It's definitely not a good idea--and you wouldn't be able to meter with the lens anyway except in stop-down mode, which is pretty inconvenient. You can easily use non-AI lenses on Nikon mirrorless cameras, as the FTZ adapters don't have the AI aperture indexing tab. They simply meter the light coming in through the lens at whatever aperture you have it set at. And in fact, I picked up some non-AI lenses in order to use them on my Z8, I never really had a use for them when I was shooting cameras, film or digital, that used the AI meter coupling system.
