The company has done precisely what it had long promised it would not.

what it had long promised it would not (do) 这是嵌入式名词性关系分句，would not 后省略了实意动词do (也可以理解为would操作词替代）。what实际上是would not do的宾语。

看一个类似例句：

My brother dances well, which I don't. (可以理解为省略了实意动词dance, 也可以理解为助动词替代。）