Starting from WorldServer 11.X, in the Modern User Interface, when clicking on Edit/Translate, you will see only the Options to open your asset in Trados Studio, Browser Workbench or Online Editor:
In the Legacy User Interface, up until version WorldServer version 11.6.1, all 5 options
Trados Studio
Browser Workbench
Online Editor
Desktop Workbench
External Editor
are enabled per default
They can - however - be easily enabled or disabled at the User Type level.
To enable or disable any of the options, go to Management > User Setup > User Types > User Type: (eg. Translator) and uncheck the option you do not want to be displayed, for instance In Desktop Workbench button for that particular User Type. This is advisable since Desktop Workbench is no longer supported.
Starting from WorldServer version 11.7.x, the options to open a Task in Desktop Workbench and in the External Editor have been removed completely both from the Modern and from the Legacy User Interface and can no longer be re-enabled. You will only find Trados Studio, Browser Workbench or Online Editor as configurable options (normally all enabled per default):