Apple have never been as clear as Microsoft regarding the timeline over which their software will be officially supported. However as a general rule the current version of the operating system, and the one immediately preceding it, can be considered officially supported. When dealing with Microsoft Office apps on iOS Microsoft are now following that premise.
Word, Excel, PowerPoint and OneNote are no longer supported for Office app updates on devices running iOS 10 (or earlier versions of iOS). In November support for Outlook will also cease for those versions of iOS.
These Office apps will continue to work, albeit officially unsupported and without further updates. Once the device is updated to iOS 11 (or later), Office apps will then resume receiving updates and patches. Users should be made aware that, if no action is taken to keep their operating system current, Outlook for iOS will eventually stop synchronising email and calendar data. Furthermore all Office apps will stop receiving feature and security upgrades.
Best advice is always to ensure your operating system is current to minimise exposure to security vulnerabilities. This will also ensure your Office programs continue to work securely too.