There is a change in support of Outlook for iOS. There are going to support the last release of iOS version to enable the modern experience to the users.
Based on thee change announcement, Outlook will support only on iOS 10 and iOS11. For more updates, check out the below message from Office 365.
Outlook for iOS end of support for iOS 9
Plan For Change
Published On : October 26, 2017
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.
Reference URL: https://portal.office.com/AdminPortal/home?switchtomodern=true#/MessageCenter?id=MC123983