The 32-bit apocalypse is right around the corner. In case you missed the memo, Apple will stop supporting 32-bit apps with the release of iOS 11. This means 32-bit apps will not open on devices running iOS 11 or later. Instead users will see a pop-up informing them that the app needs to be updated to 64-bit for it to work on their device.
While most apps will not be affected, and some developers have promised to issue updates, other developers like Plants vs Zombies creator PopCap have announced they will be pulling older titles from the App Store. Apps deleted from the App Store will still appear on your device however, so you may want to check which ones will become useless in a few weeks when Apple is expected to launch iOS 11.