Back in August, we reported Microsoft was getting ready to kill its digital assistant on Android within the year. Microsoft preferred to embed Cortana into its Office applications instead of having it compete with Alexa and Assistant. As a consequence, the app no longer works on mobile devices and can't be used to access previously created content.
Since yesterday, the Cortana app is no longer supported on Android and iOS. It can't be used to create or access reminders, tasks, and lists. These are still visible if you use Cortana on Windows. They're also automatically synced with Microsoft's To Do app, where you'll be able to retrieve the content you've previously created with Cotana.