Yeah I think you are right, I should say that some of that code was ported and still survives today in 26 uses some of the same APIs and naming, but isn’t necessarily the same code from classic Mac OS. One example is RegisterEventHotKey.

I seem to recall years ago reading that the Cocoa (the NeXTstep derived API) menu code called into the Carbon (the classic Mac OS derived API) code to run stuff. But that was around Snow Leopard IIRC. That sort of stuff might still be around?

The Carbon API itself lasted until 10.15 (Catalina / 2019) for 32 bit apps, so there's presumably at least some classic Mac OS derived code in there until then.

NewPtr() and Delay() date back to System 1.0, and they're still present in macOS 14, at least.

UI-level Carbon is gone, but various low-level APIs persist.

Pretty sure the menu bar was Carbon in Snow Leopard, which was the same release that discontinued Classic. Lots of Cocoa widgets called into HIToolbox under the hood. QuickTime wasn't fundamentally changed. And iTunes of course was Carbon code for a very, very long time.

Most of the Security framework still dates from OS 9.

But in the early days, a lot of UI and OS frameworks were based on Carbon stuff ported from Classic MacOS with Cocoa wrappers on them.

One sibling mentioned the menus. Another sibling mentioned the Security framework. The SDK for macOS Golden Gate still ships with headers dating back to MacOS 8.6 (although they are mostly deprecated now) https://kalleboo.com/linked/macos86security_vs_macosgoldenga...