When you update to or install Windows 10 Version 1803 or later or Server 2019, the Interactive Services Detection Service (UI0Detect) will no longer be present. This means you can no longer switch desktop to Session 0. Microsoft announced this change was going to occur some time ago. If you are running earlier versions of Microsoft Windows this update does not affect you. Additionally, native Windows services marked as "Allow service to interact with the desktop" via the Services applet (i.e. services.msc) are no longer granted interactive with desktop rights by the Windows Service Control Manager (SCM). Hence, even when you switch desktop to Session 0, those native interactive services will no longer be visible.
FireDaemon Pro Users
If you have FireDaemon Pro installed and you upgrade then you will no longer be able to access Session 0. Switching from the FireDaemon Pro GUI or command line will simply not work. You will need to install FireDaemon Zero and ZeroInput to overcome the removal of the UI0Detect service. Your FireDaemon Pro services will continue to run normally. Your FireDaemon Pro services will continue to be visible on Session 0 (i.e. FireDaemon Pro Interactive Services are not affected by changes to the Windows SCM).
FireDaemon Zero Users
If you have FireDaemon Zero installed, the UI0Detect service should have been preserved during the upgrade to 1803. You should be able to continue to switch desktop to Session 0. Note that installing FireDaemon Zero re-creates the UI0Detect service if it is not present.
FireDaemon ZeroInput Users
If you have installed the FireDaemon ZeroInput driver to restore keyboard and mouse on Session 0, your mouse and keyboard will still work and you should still be able to see FireDaemon Pro Interactive Services on the Session 0 desktop.