InDesign V 16.4, Current Creative Cloud as of 2021-11-14.
Currently - The top Layer visibility icon (within the layers palette) does not indicate a mixed visibility status of sub layers.
I have a rather complex layer set in a document I'm working on where I have master pages with master pages based on other master pages and linked objects within the document.
I just spent over an hour trying to locate a hidden object. I incorrectly assumed that, when looking at the layers palette, with all the layer stacks closed (layer list contracted where only the top level was shown) that the visibility icon would indicate there was mixed visibility of all layers within that stack or they would all be visible or all would be off.
When a layer is visible you have an eye icon showing. If the layer is not visible there is nothing. If the layer has sub layers where some are visible and some are hidden each level above and within the same layer stack should have an icon like an eye with a cross out or an X, Therefore when the layer stack is closed you know some of it's sub layers may be visible and some maybe hidden. When the top layer visibility is toggled there are indications for the lower levels as to there independent status and that is great but it should work in reverse as well.
This concept should also be carried out on the locking toggle as well.