It's now an optional install:

https://code.visualstudio.com/updates/vFebruary#_languages-c...

The link says: "or if you already have a project with C# files, VS Code will prompt you to install the extension as soon as you open a C# file.". As a matter of fact, it doesn't.

> As a matter of fact, it doesn't.

Actually it does. I just opened one C# file and got that prompt to install C# extension, after which everything was back to normal.

newer comment on that thread:

> This is intentional. The C# plugin is now an additional install instead of bundled with the installer. https://code.visualstudio.com/updates/vFebruary#_languages-c...