The patent application filed by Microsoft back in 2017 has surfaced online now and the document made available by the USPTO clearly shows diagrams of detachable controllers that have been designed to work with smartphones and tablets. Obviously, it might take a while for such products to hit the market and is also possible for Microsoft to drop the idea.
https://www.notebookcheck.net/Microsoft-patent-for-detachable-controllers-for-mobile-devices-surfaces-online.427222.0.html