The joystick was also a button but I don't know what options are available to assign it. My play with the D850 was quite short unfortunately.
Btw for clarity sake, I only played with the D850 but got to actually shoot motorcross at the event with a D5 and I'm extrapolating my AF joystick shooting experience from the D5. Without actually shooting a real life scenario, I might have overlooked how well suited it is for my style of shooting.