Microsoft has pulled the Cortana apps for Android and iOS from their respective app stores. The move comes after Microsoft announced that it would stop supporting its Cortana mobile apps, but it also has disabled functionality on existing installations. Microsoft recommends using its To Do app or the Cortana Windows 10 app, instead.
https://www.notebookcheck.net/Cortana-on-Android-and-iOS-is-no-more-Microsoft-recommends-using-its-To-Do-app-instead.530567.0.html