Apple will shortly release iOS 17. This is expected to happen by the end of September 2023.
At this point Microsoft will cease issuing updates for Outlook running on iOS 15 systems. Outlook will continue to run on iOS 15 devices but will not receive any feature or security updates.
In order to remain secure and compliant, ICT recommends that you update your device to the newest available iOS release for your system.
If you continue to run iOS 15 on your device after the release of iOS 17, but then update your system to iOS 16 or 17, your version of Outlook will be updated to the latest release to include any new security or feature updates. Once using iOS 16 or 17 your Outlook for iOS will continue to receive new feature and security updates.
Microsoft supports Outlook for iOS for the current and one previous major version of the operating system. In order to maintain security and feature availability in Outlook and other Microsoft apps for iOS you should keep your device up to date with iOS updates.