What do you mean by this?
The core IDE is all open source (except some closed source telemetry).
The common example I see quoted is that the super-old, legacy, C# debugger is closed source, but as far as I know there is nothing stopping people writing an alternative debugger for C# in VS Code. Jetbrains did it for Rider.
The SSH remote connection stuff, the Pylance Python LSP (you are not allowed to use it - the binary - in any other editor but VS Code, not even the OSS Codium) and some other extensions are CSS.
"""Can I load Pylance in Code – OSS? Pylance is licensed for use in Microsoft products and services only, so can only be used on official Microsoft builds of Visual Studio Code and GitHub Codespaces."""
https://github.com/microsoft/pylance-release/blob/main/FAQ.m...
Is your argument that if Microsoft develop any programming tool and then allow it to be used via VS Code they must open source it?
Would you place that same requirement on other extensions? (e.g. sonarcloud)
https://github.com/microsoft/pyright https://github.com/Microsoft/python-language-server
The only(?) one not by MS is Jedi: https://github.com/pappasam/jedi-language-server