select tt.* 
  from task_templates tt 
       left join (select ttt.task_template_id
                       , bool_or(t.published) as any_published 
                    from task_templates_tasks ttt
                         join tasks t on ttt.task_id = t.id 
                group by ttt.task_template_id) subquery on tt.id = subquery.task_template_id 
 where subquery.task_template_id is null 
    or not subquery.any_published;