I'll propose a test for whether it's true. We'll try writing instructions to a non-technical user how to perform some tasks. Then, we shouldn't know exactly which version they're using, only "I'm using OS X" or "I'm using Ubuntu." If they can reasonably muddle their way through the tasks without knowing the OS version, the OS's have remained consistent.
Example tasks would be, "run a program," "find a file on disk," "save your work," "copy and paste between applications," "close a window."
Examples that would often fail include updating configuration settings or manipulating disks. These are important tasks, but not core tasks.
Restricting Linux to the mass market distros, my limited experience is they've been reasonably consistent. They can probably use Win-R to run a program. These distros mostly use the ZXCV clipboard keys, and adopt other Windows-like conventions like Ctrl-S to save and clicking an X to close a window.
Some Linux distros could fail some reasonable tasks because of poor design; e.g. I recall Ubuntu's tiny hitbox for resizing windows being very annoying.
On OS X / macOS, Apple has chaned things up with features like autosave, which fundamentally changed how some applications behave. But according to my test, I'd just tell the person, "hit command S" and while it will create a new revision instead of saving the file, this doesn't do a thing we don't want.
I think the biggest lack of core consistency is in Electron apps and other non-native UI creeping in. I don't think I'd pin that on OS designers, though.
Between architecture changes (PowerPC to x86 to x86-64 and next up ARM), and compiler changes (I doubt Objective-C code written for OS X 1.0 will compile on the latest XCode), breaking API changes, security changes, and even deprecating standards (can't use latest OpenGL, you gotta use Metal)... not really.
Depends on how complicated it was. I can assure you that your simple GUI still mostly works as those classes came from NeXT and aren’t going anywhere soon.
