As per following link http://www.keil.com/support/docs/3901.htm?_ga=2.128675276.1303479980.1563395299-631801781.1552261077
System and Thread Viewer in MDK-ARM v5.23 and older versions doesn't support RTX v5.
As an alternative to System and Thread Viewer, you can use Component Viewer 'RTX RTOS' for RTX v5, as shown in this screenshot.
However in Keil MDK 5.17 i am seeing Missing dialog Options for Target — Debug — Manage Component Viewer Description Files option.
Is any one has success on this ?