Yes, this is about right. I was struggling to express the idea. Thank you. Win32 is an open API in a certain sense. Of course it's not open as in open source

Yes, this is about right. I was struggling to express the idea. Thank you. Win32 is an open API in a certain sense. Of course it's not open as in open source as in consensus and cooperative standards as in FLOSS, at all. But. It's well documented. Anyone can target it. It's stable. Textbooks on Windows Vista programming 15 years ago are almost completely applicable to Windows 10 development. Even software that was targeted at a reasonable subset of the Windows 95 API, whether binary or source, is still going to run on Windows 11 without change in many cases. You get notice of what parts of the API they're gonna break in the next version (usually) too. In the olden days this was an "open platform" meant, usually. It's what the open in OpenWindows, OpenUnix, OpenStep was about. An open API. As opposed to a possibly undocumented, unstable API you only used under special contract and arrangement with the supplier. Often there was no actual prohibition (legal or technical) but there'd be no support. And it'd break randomly without notice. As you say, it's a potentially enormous commitment, and organizational issue, to provide that sort of long-term API stability. And you can box yourself in regarding development and design choices.