“Export as…” was previously available as part of the SlicerMorph extension. It was recently removed from SlicerMorph because the feature is now available in the latest Slicer Preview. Since SlicerMorph specifies to the use latest version of its extension in both Slicer stable (4.11.20210226) and latest Slicer Preview that means “Export As…” is no longer available from SlicerMorph when using Slicer stable. You will need to switch to using latest Slicer Preview available from https://download.slicer.org/.