To give one example: Windows Explorer has been extensible since Windows 95. That was 21 years ago. Dropbox has to pull nasty hacks to integrate with the macOS Finder [1]. That's now.
That's not really an OS level thing, though. That's an application level thing. Finder was chosen not to be extensible, and Explorer was. There are Finder replacements that are.
OS is usually taken to mean kernel + core applications.