Thanks so much for the post.
My answer will not be to find ways to suppress of the deprecation warning. Spoiler alert.
This warning indicates that the API including your method will cease to function, and the documentation advises utilizing the AVRoutePickerView as an alternative. Consequently, I would recommend transitioning entirely to the AVRoutePickerView.
As that class you are using including the method showsRouteButton will stop working in the future releases.
Please let me know if I can help you moving to the AVRoutePickerView as the documentation shows.
Albert Pascual
Worldwide Developer Relations.