The problem is VSCode isn't open source either. Code-OSS is the open source version from microsoft.
The extensions owned by Microsoft are proprietary.
So you can't run the c/c++, pylance, remote ssh, remote containers, c#, etc extensions in open source builds of Code - OSS like VSCodium.
And for some of those, if you manually try to install the extensions using the .vsix file, they fail because the extensions are hardcoded to work only on VSCode builds signed by MS.
Pylance being proprietary is so disappointing, given it's the spiritual successor to Python Tools for Visual Studio (PTVS). PTVS was an open source Visual Studio extension, which later was ported to VSCode.
90% of PyLance is open source though, via the PyRight extension upon which its based. I'm not saying it's not disappointing its proprietary, but it's sorta inline with the "open core" philosophy that a lot of corporate open source work aligns itself with.
Ay, you're not wrong. But even granting them that, they disappointingly say “The Pyright extension for VSCode ... is not guaranteed to be ... maintained long-term.”
The C# extension is open source, and is run in VSCodium. It's a fork that just swaps out the debugger component, which is a drop-in replacement since it's a binary download. The whole stack otherwise - OmniSharp for VSCode, OmniSharp-Roslyn bindings, and the underlying Roslyn language language services - are all OSS.
The only parts of the C/C++ extension that are proprietary are parts like intellisense, which were lifted wholesale out of visual studio. Which is to say, they were never open source to begin with. All the work the cpptools extension team has done has been open source.
The extensions owned by Microsoft are proprietary. So you can't run the c/c++, pylance, remote ssh, remote containers, c#, etc extensions in open source builds of Code - OSS like VSCodium.
And for some of those, if you manually try to install the extensions using the .vsix file, they fail because the extensions are hardcoded to work only on VSCode builds signed by MS.