这句是主语从句么?This money that is shared is called a dividend.

That引导的不是主语从句,所谓主语从句是在句中作主语的,也就是谓语动作的发出者.
比如that 引导的主语从句
That he was your friend surprised me.主语：that he was your friend,谓语surprised.
你的这个句子中that引导的是定语从句,用来指定前面的money.在这里是不能省略的.如果省略变成This money is shared is called a dividend,一个句子出现了两个谓句,明显是错误的.
that引导定语从句只有在that本身在从句中作宾语时才能省略.比如：
i like the book that he gave to me.
that引导定语从句指定book,但是它本身在从句he gave to me 中是做gave的直接宾语,所以在这里可以省略掉,变成i like the book he gave to me.