Windows Phone 8.1 launched a couple of years ago and one of the best features was the Word Flow keyboard. It's one of the best native text entry systems available and in my opinion beats the iOS and Android variations.
I just read over on iMore that Microsoft is sending out solicitations to Windows Insiders offering the chance to beta test Word Flow for iOS. We have seen Microsoft release fantastic products for iOS, such as Outlook, Office, OneNote, Skype, and more, making the iPhone 6s Plus an excellent enterprise device.
At first I was surprised to see Cortana launch on iOS since Cortana is something that sets Windows Phone apart from the rest. Word Flow is another aspect that seemed destined to be reserved for Windows 10 Mobile. Given that the Lumia 950/950 XL hardware is uninspiring and the software is currently buggy, it's tough to justify a new Lumia when the iPhone runs the Microsoft apps better than the Lumia.
Word Flow was the keyboard used to set the world record for fastest text entry. Beyond the swiping support, similar to what you find on Android and 3rd party iOS keyboards, Word Flow offers an advanced prediction engine that has allowed me many times to enter full sentences by tapping on just a few letters.
I am just a bit apprehensive about seeing Word Flow on iOS because I have yet to find a 3rd party keyboard that performs reliably all the time and I always go back to the slower default iOS keyboard.