The 32-bit iss ue
Ideally before you move to iOS 11 (and while running at least iOS 10.3), go to Settings > General > About > Applications. You’ll see a list of any 32-bit apps on your device, which will stop working if you upgrade.
If there’s no update available for the versions of those apps you already own, go to the App Store and search for each app’s developer name. They might offer an upgrade as an entirely new app, which usually requires you to purchase from scratch.
Failing that, visit the developer’s website to check if the app is discontinued or has been taken on by another company. Some, such as The Omni Group, offer upgrade pricing if their new app detects an older version is installed. So, don’t delete the old one till you’ve checked for this.
If there’s no obvious upgrade path, you’ll need to seek out a replacement on the App Store.
Apps that are 32-bit and so incompatible with iOS 11 are listed in Settings.