Microsoft to end Office mobile apps support for older Android devices1
Long story short, several Office apps for Android will stop working on devices running KitKat and Lollipop effective July 2019. According to Microsoft, Office for Android will continue to work on devices powered by older versions of the OS until June, but after that, they will no longer be supported.
If you're using an iOS device instead, you're in luck. Microsoft confirmed that all Office mobile apps will work on iPads and iPhones running iOS 11.0 or later. When a new version of iOS is released, Office’s OS requirement becomes the then-current two most recent versions: the new version of iOS and the previous version.
So, there's you have it folks, if you want to continue to use Office mobile on your Android phone, make sure that it's not an old device running KitKat or Lollipop.