Yes, long overdue in the Z system. As are the broader range of custom settings
The Z9 also allows setting up 4 different i-Menus under respective custom banks. I have no experience with the D6, but it should allow similar
The D6 indeed does allow 4 different i-Menus, one for each shooting bank, just like the Z9.
So far I did not use the i-Menu at all (in any camera) as it did not appear to make too much sense to me _ i was more relying on MY MENU
What you can alternatively put in the i-menu is limited again.
Use of i-menu might change as the Z9 puts the standard metering-mode-setting only there (you can program one of the four FN buttons as well, and you have recall shooting options)
There is no more metering button- instead of D6s three buttons on the top left the Z9 has got four-but metering mode was skipped and Bracketing and Flash Button came in instead (doubtful decision). With the D6 you could configure BKT and Metering button to fulfill other functions, with the Z9 one can't.
So the general situation is that some but not all functions are in the Menu, some can only adressed in the i-Menu but not in the Menu, i-Menu cant replace My Menu, some functions are available on dedicated buttons only and it needs a high level of sophistication which functions can assigned to a Fn Button (the Z9 has great improvement here) - this could be improved.