To retrieve the IWorkspaceExtensionManager interface to unregister a workspace extension, first open the Workspace. This is not possible if a workspace extension is not installed on the client machine.
It is by design that a Workspace cannot be opened if the required workspace extensions are not installed on the client machine. Currently, however, the only way to unregister an extension is to open the Workspace to get the IWorkspaceExtensionManager interface.
Get help from ArcGIS experts
Download the Esri Support App