it was said that he had died 这句话对么?

对的
意思是：据说他已经死了
他先“死了”再据说的,所以据说用过去时,死了用过去完成时
说一个人死了,有这么几种说法：
He has died。 现在完成时的意思是动作过去发生,结果影响现在的,这里是强调“他死了”
He died (3 years ago)。 强调过去死的
He has been dead。 强调已经死了的状态
但要说一个人死了多久了,不能用die
He has been dead for 5 years。
It has been 5 years since he died。