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.