Two types of data can be transferred while using Microsoft SwiftKey Keyboard.
First, anonymous usage stats are periodically sent back (much like the Microsoft SwiftKey statistics you can see for yourself) to help us understand how people use their Microsoft SwiftKey Keyboard and how we can make it better. We get meta-level insights such as how many characters people have entered in total and how many keystrokes have been saved, but we don’t know anything about what you have been writing. If the app crashes, anonymous info about the crash will be sent to Google and to us.
Second, if you create a Microsoft SwiftKey Account, various additional data can be transferred to our servers to help us provide these services. For example, if you have Backup & Sync enabled, you give explicit permission for your personal language model data to be stored on our servers and processed to ensure prediction consistency between your devices.
All of this data can be destroyed at any time by erasing your Microsoft SwiftKey Account data or deleting your Microsoft SwiftKey Account.