The displays are apparently actually new.

[Darajavahus]

I've read here people expressing confusion why Apple is now advertising billion colors on new iMac when the previous ones some say did that already with previous system update, I right now stumbled on this apple website where it's now distinguished between them stating clearly that displays from 2014 and 2015 support millions of colors and 2017 displays support one billion colors, so that seems to make that this isn't software but actually new hardware. It looks like also these are first ever apple displays that support billion colors even though they're still not true 10 bit but dithering. Sorry for my poor English.

https://support.apple.com/en-gb/HT202471

 

[redheeler]

No, these are not the first, as I have said on here before. The original 5K iMac released in 2014 was the first (wasn't supported until the release of OS X El Capitan, however). Apple simply never chose to advertise it until now.

4K and 5K iMacs Support 10-Bit Color Depth on OS X El Capitan