Currently, Fire Emblem Heroes supports iOS 13.0 or higher. After the next update to Ver. 9.0.0, however, the game will require iOS 15.0 or higher.
After the update is released, players using an iOS version older than iOS 15.0 will no longer be able to download, update, or access the game.
If you will be affected by this change, please consider updating your device's iOS to version 15.0 or higher if possible.
Note: If you plan to change to a new device, please back up your save data using your Nintendo Account to move it to another device. Please note that even if you link your save data to your Nintendo Account, you cannot transfer your Orbs, Forma Souls, Otherworld Bonds, and some other items to the other device if you are transferring your data to a different OS.
■ See the following for instructions on how to link your save data.
How do I transfer my save data? (When changing to a new device, etc.)
Related questions: Can I use the same Feh Pass Subscription even after changing my device to one with a different OS?