It has not been decided who will be sent to carry out the task.
这是类似于it is done that,也有it is done who?
It 是形式主义,has not been decided 是谓语(被动语态否定式)who will be sent to carry out the task是状语从句作真正主语。.如:
It is still open to question who is to head the group. 谁领导这个小组还未决定。(赵俊英:《现代英语语法》p.1219)