Mimicking cascading foreign key referencing system table
If you are just looking for a way to get ids of the droped table, maybe this code will help you out :

CREATE OR REPLACE FUNCTION master.ddl_trigger_drop_table_fct()
  RETURNS event_trigger
 LANGUAGE plpgsql
   AS $$
DECLARE
    all_variables character varying;
    r RECORD;
    schema_name varchar (250);
object_type varchar (100);
BEGIN
    FOR r IN SELECT * FROM pg_event_trigger_dropped_objects () LOOP
        if r.schema_name = 'master' and r.object_type = 'table'
         then
            schema_name := r.schema_name;
            object_type := r.object_type;
            all_variables := concat(
                'classid: ', r.classid, chr(10)
                , 'objid: ', r.objid, chr(10)
                , 'objsubid: ', r.objsubid, chr(10)
                , 'original: ', r.original, chr(10)
                , 'normal: ', r.normal, chr(10)
                , 'is_temporary: ', r.is_temporary, chr(10)
                , 'object_type: ', r.object_type, chr(10)
                , 'schema_name: ', r.schema_name, chr(10)
                , 'object_name: ', r.object_name, chr(10)
                , 'object_identity: ', r.object_identity, chr(10)
                , 'address_names: ', r.schema_name, chr(10)
                , 'address_args: ', r.address_args);

            RAISE NOTICE '%', all_variables;
        end if;
    END LOOP;

    ... [do some stuff] ...
END;
$$;


and after you have adjusted this code to fit your needs, you just need to bind your function too your event trigger :

CREATE EVENT TRIGGER ddl_trigger_drop_table ON sql_drop WHEN TAG IN ('DROP TABLE')
   EXECUTE FUNCTION [schema_name].ddl_trigger_drop_table_fct();


But same as others : why do you want to do this might interests me a bit ;)