The current Input Method Framework does not exist in the application form and IOS system, but is dynamically called in the form of system components. You can input the UI component in any third-party app to call the Input Method Framework.
The reason for not enabling the input method is obvious. For all third-party apps, Apple's attitude is to hope that everyone can run independently in their respective sandbox. All interactions are achieved through the API framework and components provided by the system, in this way, every application running on IOS system can obtain the same permissions and resources without interfering with each other.
Currently, the design of the calling method of the Input Method in the system does not consider the issue of independent sandbox at all. If a third-party Input Method crashes, the app that calls it will crash. This is unfair to other apps.
Based on this, if Apple does not re-design the call method of the input method, I think the possibility of enabling the IOS input method is not high.