As of August 2018, the ProtonMail iOS app no longer provides support for iOS 8. This is because Apple no longer supports iOS 8, and therefore security vulnerabilities are no longer being patched.
If you are still running the ProtonMail iOS app on iOS 8 you can continue using the app, but you will no longer get updates, new features, bug fixes, or security patches.
For those still using iOS 8, we recommend updating your system to iOS 9. Apple provides instructions to do so here: https://www.apple.com/lb/support/ios9/.