Google has decided to focus its efforts on the AdSense mobile web interface and discontinue the mobile apps. Those using AdSense on mobile devices were probably expecting this, since the Android app has not been updated since December 2017 and the iOS AdSense app received its latest refresh on November 2017.
https://www.notebookcheck.net/Google-discontinues-the-AdSense-apps-for-Android-and-iOS.427692.0.html