The wiki article for it in the context of software (https://en.wikipedia.org/wiki/Proprietary_software) seems to pretty much stand it in opposition to open licenses, but I also have a sort of gut feeling that it could also refer to interfaces in OSS that are not designed to re-implementable. Not sure about that though.
In the context of not software licensing, I would define it roughly like you said. Interfaces that are designed in a way that is not compatible with existing software or easily implemented by others.
An example would be QT, the GUI framework for C++. It has it's own implementation of stuff that already exists in the standard library, like the string container for example (std::string). You can't use standard C++ types with QT and you can't use QT types with non-QT C++ libraries or types.
I am not a C++ or QT expert so there may be some level of compatibility between the types when using generics or in general, but from my very tiny use of QT it seemed like you had to use their types.
XBL, a precursor to Web Components in use in Firefox for 15+ years before any ordinary webdev was talking about shadow DOM, was open source in every sense, for example. It was also proprietary.
See also: "Problems with XUL".
> XUL is a proprietary technology developed by Mozilla and only used by Mozilla.
I don't know when that page was written, but there were other developers that were using XUL before it was deprecated. The only one that comes in mind right now is ActiveState and their Komodo Edit/IDE. But there used to be more developers and I recall their being a dedicated page that listed those other products built on XUL.
The documentation of it, and the jump in complexity when it came to XPCOM (which were written in C++), were the reason (in my opinion) why platforms like Electron got popular instead of XUL.
