This was a very simple and non scientific test the D750's sensor at various ISO settings, from 50 to 51200.
Lighting was simple, a single lamp placed to the left and 45 degrees from the target.
I used a 50mm f1.2 AIs Nikkor and the self timer. I did some previous testing to see if mirror lock up improved the image, it did not. Exposure times were roughly 1/8 to 1/4000 all at f4.
At a later date I will repeat this test on a outdoor subject of various colors. On the full frames I noticed vertical banding appearing at ISO's above 12800. I would conclude this body is useable to ISO 12800 but goes down hill above that. View at original size.