But the configuration of what marketplace is used and what extensions have access to the privileged "proposed API" makes huge difference in what Microsoft/GitHub can provide as user experience and what other developers can do.
See this great post for detailed explanation how Microsoft created a closed ecosystem around VS Code https://news.ycombinator.com/item?id=32657709
I'm perfectly aware of what Microsoft could do in some distant future. But as is, for all intents and purposes, VSCode is open source
... Except that if you use the source to make your own build, you lose the extension ecosystem that is one of VSC's major selling points. It is at best a worked example of tivoization.
rm -fr ~/.vscode/*
cp -a ~/.vscode-oss/* ~/.vscode/
# Run VS Code, download extensions
cp -a ~/.vscode/* ~/.vscode-oss/
Based on my recollection of migrating to VSCodium, all my extensions continued to work just fine.I think there's also a hacky way to let VSCodium access the VS Code extension marketplace, but I believe it's an EULA violation.
VSCode is also completely open source. The only thing not "open" about it is the keys they use to make the final distributable. Even the VSCodium docs say as much. They literally just build the final distribution but with slightly different keys to point to their open source extension distribution website instead of the Microsoft one