No official announcement has been made for this move but Microsoft accidentally released build 16212 for Windows 10 and phones and it includes a number of the features that are necessary for on-screen inputs. As MS Poweruser explains, one of those features was Composable Shell and one of its components is TextInput, which is the basis for the Windows keyboard.
The purpose of Composable Shell, or CShell, is that it can scale Windows 10 to the device and display type you are using. Using technology developed by Microsoft acquisition, Swiftkey, the on-screen keyboard should be able to work with any device of any configuration, as long as it is running Windows 10. It will also support swipe inputs, much like the Windows Phone keyboard.
While none of this will be of huge interest to desktop users, those using tablets or 2-in-1 systems will likely see some benefit from it. The Windows Phone WordFlow keyboard is well liked by Windows Phone users, so could find a much larger receptive audience on Windows 10 tablets.
Unfortunately, though, nobody knows when we can expect to see this feature added to a new Windows 10 build for consumers to enjoy. As of now, it is believed the internal build was rolled out sooner than planned to Windows Insiders on the Fast ring, so it may be some time before it is leaked out further to the Slow ring.
However, Microsoft has not even acknowledged that the text input feature exists yet, so we will likely need to wait for that to happen before making any major speculation about when we can expect to enjoy it. Until that happens, know that it will need to proliferate through the Insider rings before seeing the light of day.
