There are also a whole bunch of APIs left out of 64-bit macOS, including almost all of Carbon that dealt with UIs (HIToolbox, ATSUI, QuickDraw, Nav Services, Dialog/Menu/Font Manager, etc). None of these APIs have received anything more than bare maintenance in the last 10 years, and evolving the system while keeping them operational has not been easy.

In a way, this is the true last step of the Mac OS X transition that started in 2001--the removal of the transitional APIs that made it possible.