As of July 2019, the ProtonMail iOS app will no longer support iOS 9. This operating system is four years old and contains security vulnerabilities that are no longer being patched by Apple. Therefore we are only developing new features for later versions of iOS.
If you are still running the ProtonMail iOS app on iOS 9 you can continue using the app, but you will no longer get updates, new features, bug fixes, or security patches. Version 1.11.10 of our app will be the last version that supports iOS 9.
For those still using iOS 9, we recommend updating your system to the latest version of iOS. Apple provides instructions to do so here: https://support.apple.com/ios.