It was disallowed for other Microsoft products to call undocumented APIs. Anything called by Office/Azure/whatever needs to be a publicly documented API, and t

It was disallowed for other Microsoft products to call undocumented APIs. Anything called by Office/Azure/whatever needs to be a publicly documented API, and there are automatic checks in all Microsoft codebases to confirm that no undocumented APIs are called. Windows components, of course, aren't subject to any such rules. There have always been and always will be interfaces necessary for Windows to call itself that the company has no interest in supporting in a backwards-compatible way and publicly documenting. An example is pinning applications to the taskbar: Windows needs to be able to do it, but if it was a public API every app would do it and the experience would be ruined. Of course, Chrome eventually figured out a way to bypass and do it (I believe using accessibility hooks to simulate user input? I forget the details), at which point the arms race escalated from there - the Windows team added new protections in that area - but I haven't worked in that area in a long time and don't follow it in detail.