Current C# plugin (Omnisharp) is opensource, so they announce that it will be replaced by a closed source selector that let you choose the opensource or the proprietary plugin with the proprietary as default.
It's Embrace, Extend, Extinguish just in front of our eyes, so opensource people are pissed.
Most of the heavy lifting in OmniSharp is done by Roslyn [0]. Roslyn is the open source C# compiler and code analysis library maintained by MS. You cannot overstate how much effort has gone into Roslyn, and how much money MS has invested to make it work. OmniSharp is essentially a wrapper[1] around Roslyn (before Roslyn it was Mono), and while the OmniSharp team did a nice job developing a nice intellisense UX around it all, it is not like MS is taking it all away from them. Incidentally, it is almost certain that new intellisense plugin by MS will also be built on top of Roslyn.
[0] https://github.com/dotnet/roslyn [1] https://github.com/OmniSharp/omnisharp-vscode#the-c-extensio...
The builds are proprietary.
It’s based on an open source core but the builds can include anything that Microsoft thinks will be commercially useful to it.