Officials have not confirmed how many of the more than 50 people originally due to be deported were actually on aboard. =

Officials have not confirmed how many of the more than 50 people who had been originally due to be deported were actually on aboard.

划线部分是 people 的定语。意为“原定/理应被驱逐出境的人”。

be due to do sth. 预定/原定/理应做某事。

主要结构：

Officials （主语）have not confirmed （谓语动词）how many of the more than 50 people (originally due to be deported) were actually on aboard.（宾语从句）