You can use the parameter geo=false. However, by default this includes observations with geoprivacy=private and/or taxon_geoprivacy=private. So you also need to append verifiable=false and I think that will get you all the observations truly missing locations.
https://www.inaturalist.org/observations?geo=false&verifiable=false
1.2 million observations from over 287K user accounts… Yikes.