It was realized more or less in Mac OS X Server 1.0, which despite its name is more Rhapsody than Mac OS X, and enough people bought it to complain when Apple quickly moved ahead with Mac OS X as we know it and didn’t offer any kind of discount on Server 10.0.

EDIT: Not the best quality, but here’s a demo I found on YouTube for those interested: https://youtu.be/HrHNub_vJtA

Basically imagine NeXTSTEP/OPENSTEP, but with a Platinum skin, the window server is still a Display PostScript interpreter, there is a Classic Environment in the Blue Box, and instead of the Finder you have NeXTSTEP’s Workspace Manager and its still App Kit and Foundation Kit. Carbon isn’t a thing yet, or it’s in early development and hasn’t been announced yet.

I don’t recall if the kernel had replaced Driver Kit with IO Kit yet though.