Navigate to All > Now Experience Framework > Experiences.
Select the application you want to add to the Workspaces menu on the Unified Navigation.
Note: A warning message appears if you are in a different application scope than the application form you are viewing.
Application scope warning message example
To change your application scope
Select from the header bar.
Select application scope.
Search and select your new application scope.
In related lists, select the UX Application Category M2Ms tab.
Select Workspace in the Experience Category column.
be careful deleting - The KB article I found on SN Support suggests the same approach as suggested here by Abdul Khader - with the important added detail: 'take a backup of these records' before deleting. (I learned the hard way)
KB1201798 Need to Hide Workspace Tab in the Unified Navigation in Next Experince
In case you want to hide the Workspace category in general - you can navigate to the Unified Navigation Menus [sys_polaris_menu_config] table and just set the Workspaces record - Menu active = false