I have the Z6 and as other stated the blacks are not crushed at all on the Z6. Everything is recoverable. I will post sample footage from test I did yesterday that proves this as a fact. Nikon does give the user more dark shadow area out of the gate in photo and video. I didn't raise the exposure in my test but it also didn't matter if you did over expose the shot. I did find out that the image comes out pushed to the lighter side then what you see on thru the view finder or back panel. The issue you are seeing (like the S1) is that Nikon's secret sauce settings are not anywhere close to a flat profile and they follow there still image look. So based on my test you have to shoot with the flat profile and the -2 contrast and -2 brightness and 0 sharpness. Again the shadow / crushed blacks have every single bit of data in them and are not baked into he footage.
Even when I shot closer to standard or even auto highlights and crushed blacks were recoverable meaning I lowered the highlight and raised the shadows and used mid tone to balance the shot.
My conclusion is you have to treat the Z6 like the FS7 cine camera and almost under expose your look, almost, more then you think and more then the viewfinder is showing you.