P: Filter by Unknown Camera

There used to be a selection under filter by camera for "unknown camera" to see all pictures that are, for example, screenshots uploaded from my phone.  Now it's gone, for some reason, and I'm unable to filter all the stray images in my 17k photos that aren't real camera photos.

Was this intentionally removed?  Can we have it back?  Is there a workaround to see al images that don't have a known camera in the metadata?