If I go to Settings > Taskbar Settings > Turn System Icons On or Off, the switch for Action Center is grayed out. If I flip it off & on again, the Notifications button still does not appear on the Taskbar.
If I go to Settings > Notifications & Actions, the Notifications switch is turned on. I'm using Horizon Client version 5.5.2 build 18035009.
The Windows 10 notifications Taskbar button and Windows Action Center are missing from my Horizon sessions, and despite all attempts they cannot be made to appear.