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Material's Ambient Occlusion input....

Has this been deprecated? Curious what function this serves at the moment.

I doubt its deprecated.
It should serve the ambient occlusion…

what is ambient occlusion?
essentially fake light bounce information.
for global illumination (the skylight).

If you have ray tracing on, this material setting is probably not applicable, since .22 when real time Ray traced ambient occlusion was introduced.
but a shader will probably still compile it as a fallback if something is plugged to the input.

Let’s say that its a semi-useless pin if you are using dynamic lights ON flat geometry.

On complex geometry this pin becomes important again.
imagine a character with a helmet. The helmet has glass covering the eye hole.
You can use an AO map to darken the eyes so that they won’t render as bright when the helmet is on.
Likewise any geometry like crevices or similar that isn’t necessarily defined by geometry (think of a complex crevice normal map) can benefit a lot from using a matching AO map.
then again, you probably want bent normals on a character… but thats another topic.

Hi, if you use dynamic lightning then the ambient occlusion input in the material will do absolutely nothing. It is only used during the building of static lightning (that was the state how it was working when they created the Master Materials Learning Course, and as far as I know this has not changed).