https://techcrunch.com/2018/10/12/facebook-messenger-unsend/

In April, TechCrunch broke the news that some of Mark Zuckerberg’s Facebook messages were deleted from recipients’ inboxes in what some saw as a violation of user trust. Then, Facebook suddenly announced that it would actually build this Unsend functionality for everyone. Then six months went by without a peep about the feature, furthering suspicions that the announcement that it would release an Unsend button was merely a PR driven response to the scandal.

Facebook retracted Zuckerberg’s messages from recipients’ inboxes