Same for the c# one I think, the old language server was and is still open source but the .net core debugger has always been proprietary.

I imagine it is because it is derived from the Visual Studio debugger in some fashion. JetBrains ran into the same problem with Rider back in the .NET core days and had to write their own debugger.

Roslyn language server is OSS too: https://github.com/dotnet/roslyn/tree/main/src/LanguageServe...

There was never an issue with Omnisharp OSS-ness itself nor what replaced it. It was always about debugger and then "Dev Kit" extension which builds on top of the base one - "Dev Kit" is what isn't OSS and what requires an account.

There is also an alternate open debugger: https://github.com/Samsung/netcoredbg / https://github.com/muhammadsammy/free-vscode-csharp (extension fork which swaps vsdbg out for this one)