Techinsights has just analyzed that the Nikon Z9 (Amazon/B&H Photo Video/Adorama) ‘s 45.71MP full frame sensor is Sony IMX609AQJ.
From Techinsights:
This report presents a device essentials folder (DEF) of the Sony IMX609AQJ full frame 45.7 MP stacked back-illuminated CMOS image sensor from the Nikon Z9 mirrorless camera.