>It sounds like the C/C++ extension was proprietary

The extension itself it MIT licensed (so could be hosted on the open VSIX store, if it wasn't down because the Eclipse project is suffering from server issues right now). In theory any fork can patch out the check and re-release the extension.

However, the extension packages some binaries that are proprietary, and have been since about four years ago. People could re-implement those and re-release an open version of the extension, but you can't just (legally) take the proprietary binaries and ship them if you don't have the license.

Open alternatives actually exist, but their quality and ease of configuration depends on your use case. In large projects the proprietary extension seems to be worse from what I've read.