MonoDevelop 5.9.6 removed from Unity Installers and support for it has been deprecated in Unity.
MonoDevelop 5.9.6 has been replaced by Visual Studio for Mac on macOS as the bundle C# script editor in the macOS installer. Visual Studio 2017 Community is now the only C# script editor installed on Windows.
Unity no longer recognises MonoDevelop as the “MonoDevelop (built-in)” external script editor in preferences, when it is installed in the default location next to the Unity executable. When no C# code editor is installed and selected in preferences, Unity uses the system default application for opening C# (.cs) scripts.
так что. по сути инсталер от Юнити - галочка установить визуал студию и работать в ней