As reported by TechCrunch, the useful feature seems to be unannounced. TechCrunch notes that Facebook has confirmed the feature - reportedly dubbed the "Like Reply" button - saying that it was part of a recent update, albeit it was not listed in the official change log.
The said feature is now available on Facebook and Messenger for Android and iOS. This new feature will definitely save a lot of time. Just "Like" a message to hint an affirmation. No more writing lengthy replies or thinking about what to say next. Voila!