C# extension works well and uses Roslyn Language Server that is part[0] of the Roslyn (C# compiler) itself - this is what the base C# extension[1] uses. Both of these are licensed under MIT.
The closed-source part is 'vsdbg' which is Visual Studio's debugger shipped as a component that the extension uses. It, however, can be replaced with Samsung's 'NetCoreDbg' by using the extension fork[2].
[2]: https://github.com/muhammadsammy/free-vscode-csharp (please consider giving it a star, it's the only actively maintained alternative and other tools end up relying on it downstream to support debugging - VSCodium as well as Emacs and Neovim with VSC extension bridges)