[brassic]

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.

[1]: https://news.ycombinator.com/item?id=12463338

[st3v3r]

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.

[paulddraper]

OS is usually taken to mean kernel + core applications.

Are Explorer and Finder "core"? Probably.