It has been pointed out in the comments that this could possibly still be part of Facebook’s test because some users say that they had the feature before it was removed, hinting that the feature’s availability is enabled or disabled server side. There is no word on when the feature is expected to become available to all, but we’ll keep our eyes peeled for it.
For those unfamiliar, the unsend feature is as its name implies. It will allow users to unsend messages to other users. This came on the heels of how some users were discovering that messages they received from Facebook executives had been deleted. Facebook later revealed that they were working on an unsend feature that would let users do the same.
When the feature does eventually become available to all, it will let users unsend messages that they have sent in the last 10 minutes. Some have complained that the window is a bit too small, especially with WhatsApp (also owned by Facebook) giving users an entire hour to unsend a message.