A 1972 agreement between Canada and the United States reduced the amount of phosphates that municipalities had been allowed to dump into the Great Lakes.
OG的解释是,因为dump 这个动作在修改了法规之后仍然存在,所以用 had been 是一个不正确的时态。
我个人觉得 had been是表明allow 的时态,用 had been allowed 与reduced形成对比,体现出,这个allow是在reduce之前的,我觉得其实比用 were更合理。
可是,如果用had been,过去完成时,暗含意思动作协议生效那天前已经结束;然而事实上agreement just changed the amount of blahblah 而不是终止该动作,该动作在协议达成并予以实施后仍然进行,此时从句用一般现在和一般过去都行,至少要和主句时态reduced一致,或是时间点晚于一般过去时态的一般现在时态,DEAR。