Word Flow was an alternate keyboard for the iPhone that arranged the keys in an arc so they were all easily reachable with your thumb. That made it possible to use your iPhone one-handed for typing even on the iPhone 6 Plus and iPhone 7 Plus.
Microsoft classified Word Flow as a Garage project, which means it was always an experiment of sorts. The Word Flow Garage webpage now states,
The Word Flow experiment is now complete! We encourage you to download the SwiftKey Keyboard from the App Store. The SwiftKey product team is frequently building and evaluating new features for SwiftKey and shipping updates.
Along with the Arc Mode keyboard layout, Word Flow also learned your typing patterns for better predictive typing, suggested names based on your Contacts, offered web searching, and could find GIFs, emoji, and images.
SwiftKey does much of that already, plus it includes the popular swipe typing method. It doesn’t, however, offer Arc Mode which means Word Flow’s killer feature is—at least for now—gone if you didn’t already have it installed.
While Word Flow’s demise is unfortunate, it isn’t too surprising. Microsoft is clearly focusing more on SwiftKey and supporting two independent smartphone keyboard projects doesn’t make sense. Hopefully we’ll see Word Flow’s Arc Mode keyboard in a SwiftKey update soon.