In Short: It seems like Facebook is pushing out Messenger redesign to some users, presumably testing the change before pushing it to everyone. According to the source, this update appeared out of nowhere, without an update or anything, presuming this is only a server-side change that Facebook pushed out to some users. The app itself looks considerably different, as you can see in images down below. The bottom bar now has only three tabs, while the camera and new chat button got repositioned to the top right corner. Games, businesses, and a bunch of other secondary features have been moved to the 'Explore' tab on the bottom. Chat screen is also changed in this version of Messenger, it looks even cleaner now. It's quite obvious Facebook is not exactly following Google's Material Design guidelines here, iconography is a bit odd, and it just doesn't look as one would expect from such a popular Android app, but that's not exactly surprising, as the company announced this design a while back.
Background: Facebook first talked about this redesign back in May, during its F8 2018 conference, and the company did say that the update will arrive to smartphones soon, but did not specify when. Well, we're in September already, and the update is still not here, it seems to be in testing still. Now, aside from the interface change itself, this update is also expected to bring Dark Mode to (Facebook) Messenger, at least that's what Facebook promised back in May.
Impact: This redesign may not appeal to everyone, but it's hard to argue that it looks a bit cleaner than the current version of Messenger. It remains to be seen how long will it take Facebook to make this update official and start pushing it out to every Messenger user. Once that happens, we'll see how will the community react to the change, and if the Dark Mode will be a part of the package, as Facebook promised.