I keep a memento personal page with the salient specs of the flagship Nikon models, I recently updated the page with the Z9's features:
1999 D1 Original model, 2000x1312, 2,7 Mpix, ISO 200-1600 4,5 fps, 1/16000"
2001 D1H FPS up to 5 fps
2001 D1X Resolution upped to 3008x1960, 5,3 Mpix (still 1/16000")
2003 D2H FPS upped to 8 fps, 4,1Mpix, 2464x1632 (shutter speed down to 1/8000")
2004 D2X Resolution upped to 4288x2848, 12 Mpix, ISO max 800, 5 fps
2005 D2Hs Added ISO Max 6400
2006 D2Xs Added ISO Max 3200, High-speed Crop Mode
2007 D3 FX, 4256x2832, 12 Mpix, 9 fps, ISO MAX 25600
2008 D3X 6048x4032, 24 Mpix, ISO Max 6400, 5 fps
2009 D3s 4288x2848, 12 Mpix, ISO Max 102400, 9 fps
2012 D4 4928x3280, 16 Mpix, ISO Max 204800, 10 fps
2014 D4s ISO Max 409600, 11 fps
2016 D5 5588x3712, 20,8 Mpix, ISO Max 3280000, 14 fps
2020 D6 EXPEED 6, improved AF, GPS included, added 900" in M mode
2021 Z9 EXPEED 7, 1/32000", 1/250" flash sync, stacked sensor, improved AF, 20-120 FPS, 8256x5504 (45.7Mpix), 8K video, infinite shutter life. Did I forget something?
I have mentioned a feature only when there is an improvement or feature addition over the previous model. GPS and 900" features were not included in the Z9's list, because they were already present in the D6. Z9 has a lower ISO MAX than the D6, but it can focus in almost total darkness (probably improved over the D6).
Naturally, improvement is a continuum, with quantum leaps at the D3 point (introduction of FX sensor, 25600 ISO) and Z9 (changing the game all over). The initial D1 leap (birth of pro digital camera in the Nikon world) should be accounted for.
Ciao from Massimo