Apple is planning on adding TouchID and Face ID in iPhone 2020, according to Bloomberg. At the same time, it is stated that the feature can also be transferred to the iPhone of 2021. With the arrival of the Touch ID below the screen, you would have the option to unlock the iPhone with Face ID.
Bloomberg further report states that Apple already has a prototype that would make this possible, but more work is needed to make it happened. The release of the iPhone 11, which is expected later this month would, therefore, come too early for the new feature.
By the way, there will be a cheaper iPhone in 2020, which will be marketed as a sort of iPhone SE. The design would look a lot like the iPhone 8 with a Touch ID home button, as we are used to from the older models. The Touch ID under the screen would be something for the expensive models within the iPhone family.