https://msdn.microsoft.com/query/dev16.query?appId=Dev16IDEF1&l=EN-US&k=k(-_CSharpKeyword);k(DevLang-csharp)&rd=true
Currently, this just redirects to https://docs.microsoft.com/en-us/
I don't know if it's a temporary problem or not. I see that f1_keywords for "-_CSharpKeyword" already exists in subtraction operator article. The issue reason isn't clear to me.