Microsoft's recently announced beta test of its Word Flow keyboard for the iPhone reportedly includes a new one-handed interface. That kind of UI is not included in the Windows Phone or Windows 10 Mobile version of the keyboard.
According to Thurrott.com, the UI is designed to look like a fan, with all of the keys placed on the grid. The layout is made so that users can type on the keyboard just by using their thumb, according to the report. While the Windows 10 Mobile version of Word Flow has its own one-handed mode, it's in the same kind of layout as the regular version, but just made smaller and shifted to one side.
The report does point out that the iPhone beta of Word Flow could ditch the fan-based UI when it is officially released. There's no indication that this one-handed interface will be offered to Windows 10 Mobile users in the future.