这句英语，主语是topics吗？主谓宾或者主系表都是什么，从句？帮我分析一下吧？

Topics such as death, for example,were once considered so upsetting and unpleasant that it was a taboo to even talk about them.

整个句子直译过来大意是：
诸如像死亡这样的这样的话题曾经被认为是如此的令人苦恼和不愉快以至于谈论它们（指代像死亡一样令人不愉快的的话题）成了一种禁忌。

我一一帮你分析一下啊：

首先从大的方面看这是一个so...that...句型，我们明显在句中能找到so和that，所以整个句子是一个表因果关系的从句，有两个句子，一个表“因”，一个表“果”；

然后我们详细分一下整个句子的句子成分：

*“Topics such as death, for example,were once considered Topics such as death, for example,so upsetting and unpleasant”这是整个句子的前个句子，它表“因”是个被动语态的句子。
其中主语是“Topics such as death, for example,”topics是中心语，such as death修饰topics，只是举个例子罢了，for example是插入语！
“were considered”是谓语；
“once”是时间状语，表过去；
“so upsetting and unpleasant”是宾语补足语，因为这个句子是被动的，正常语态应该是consider Topics such as death, for example,so upsetting and unpleasant，“so upsetting and unpleasant”是作为“Topics such as death”的补足语。

that只是两个句子的连接词没什么实际意义。

*让我们看后半个句子“ it was a taboo to even talk about them”，是个主系表结构：
it 是形式主语，真正的主