Due to the rapid changes of mobile device OS and Office 365 specifications, Microsoft release a communication to inform us to update your Apple devices to a new iOS version to have a better Outlook for iOS compatibility:
With the release of iOS 11, we want to remind you that Outlook for iOS is supported only in the last two releases of iOS. This means new app updates to Outlook will now only be supported on devices with iOS 10 or iOS 11. How does this affect me? You are receiving this message because our reporting indicates one or more users in your organization are using Outlook for iOS on iOS 9. Devices with iOS 9 are no longer be supported. While older versions of Outlook will still work on devices running iOS 9, these devices will not get new app updates after build 2.48. Users can expect a degraded experience of the Outlook for iOS app over time, if they do not stay up-to-date. What do I need to do to prepare for this change? We encourage you to stay up-to-date to maintain the best Office 365 experience.