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)