Many important plugins are only in the official marketplace, and it's not allowed to use this marketplace from open source builds.
The practical effect is that open builds like VSCodium don't have access to things like the C# plugin, making them not useless, but much less viable than actual VS Code.
C# has a fork of the official plugin which uses NetCodeDbg by Samsung. And the language server itself is a part of the SDK anyway. It works in VSCodium without any additional effort required.
I didn't know that it isn't allowed?
VS Codium even endorses downloading the files from there and installing them in Codium. Is this against their TOS or something?
The practical effect is that open builds like VSCodium don't have access to things like the C# plugin, making them not useless, but much less viable than actual VS Code.