With the release of iOS 11, Apple dropped support for apps built using an older technology known as 32-bit. Many of these apps have not been updated in 3+ years and may have had some performance/usability glitches already. Beginning with iOS 11, they will no longer work at all unless the developer updates them.
Finding Incompatible Apps
The main way people find incompatible apps is by launching them--apps that can't run on iOS 11 will display a warning that the app needs to be updated.
Another way to locate all the incompatible apps at once is to open the Settings app and navigate to General→About→Applications. If incompatible apps are present you'll be able to tap on the Applications menu to see the list. If all apps are compatible, the Applications menu will just show the total number of apps.
When you encounter an incompatible app, please submit a Help Desk ticket using the custom ticket type "iOS 11 - Incompatible Apps". The Technology Department will contact the app developer and post updates in the ticket and in the iOS 11 Incompatible App Tracker.