The SwiftKey personalization service, which is a feature of SwiftKey Cloud, accesses your recent content from online services that you specify, such as Gmail, Facebook and Twitter. It uses this content to build a personalized language model on our servers, which is then transferred to your device. This is an optimized view of the words and phrases that you use most often, and reflects your unique writing style. Your use of our personalization service means we may store and use data provided by you to develop and improve our Products. You have the right to have this data destroyed, as outlined below.
I have a theory that it's impossible to prove anything, but I can't prove it.