SlicerLookingGlass development has not kept up with updates to newer versions of VTK which it uses. You’re not alone in wondering about the status of this extension. See other post:
Currently with the latest Slicer stable (5.6.1) the SlicerLookingGlass extension is not building successfully across all platforms (Windows, macOS, Linux). See build details here. These build errors are why SlicerLookingGlass cannot be found in the Extensions Manager.
@jcfr has been the primary developer of SlicerLookingGlass and may be able to respond about which version of Slicer was the extension last working. If he does not respond, you can write up an issue in that repo to document the state of it not working with latest Slicer.