Here's the announcement:
To make your personal digital assistant as helpful as possible, we’re integrating Cortana into your Microsoft 365 productivity apps. As part of this evolution, on January 31st, 2020, we’re ending support for the Cortana app on Android and iOS in your market. At that point, the Cortana content you created–such as reminders and lists–will no longer function in the Cortana mobile app or Microsoft Launcher, but can still be accessed through Cortana on Windows. Also, Cortana reminders, lists, and tasks are automatically synced to the Microsoft To Do app, which you can download to your phone for free.
After January 31st, 2020, the Cortana mobile app on your phone will no longer be supported and there will be an updated version of Microsoft Launcher with Cortana removed.
For now you can still download Cortana from the U.S. App Store. We'll let you know if we learn any more details.