Improving row estimates for query with jsonb_object_keys()
A super-user can do:
alter function jsonb_object_keys ( jsonb) rows 2;

But this will change the estimate for all uses of jsonb_object_keys in that database, not just that one use of it.
If you don't like that, you could duplicate the function and change the duplicate:
CREATE OR REPLACE FUNCTION jsonb_object_keys2(jsonb)
 RETURNS SETOF text
 LANGUAGE internal
 IMMUTABLE PARALLEL SAFE STRICT ROWS 2
AS $function$jsonb_object_keys$function$;