The iNat api docs list user_id twice under Identifications (actually three times – two are valid), but the description suggests one should actually be place_id. Trying to query by place_id doesn’t produce the expected filtered results, e.g. https://www.inaturalist.org/identifications?place_id=21 doesn’t filter to place_id 21.
Looks like @pleary fixed the place_id thing, fix for user_login forthcoming. Thanks for the heads-up. Regarding https://www.inaturalist.org/identifications?place_id=21, that’s a different API. The docs at https://api.inaturalist.org describe endpoints at that subdomain, e.g. https://api.inaturalist.org/v1/identifications?place_id=22, which does return identifications made on observations from place 22 (Idaho, US).