HIView and HIWindow were added, if I recall correctly, because it allowed other toolkits (tk, Qt (who only recently moved off this), etc) to draw the native look “properly”. It was never actually up to date or well documented, and they kept it around just so the OS didn’t look so totally bizarre as you moved through it (like you see on Windows).

Bolting on Retina support to that is such a no-brainer that I hesitate to call it adding features - that was table stakes in making sure the switch to retina didn’t look like total ass.

None of this stuff ever changed how Carbon was deprecated. People just sat around not listening to Apple, and then the last two years the bigger GUI toolkits finally did the work to transition properly.