Good news, everyone!
KeePassium is the first KeePass app for iOS that supports YubiKey.
KeePassium adopts the challenge-response approach used by KeePassXC. Firstly, because their approach requires only the database itself — whereas KeeChallenge and OtpKeyProv rely on auxiliary files (which would complicate synchronization). Secondly, because KeePassXC is available on all major platforms as a native app.
For more details: How to use YubiKey with KeePassium/KeePassXC.
(The App Store release version of KeePassium does not yet support YubiKey 5Ci — this requires a special permission by Yubico and Apple.)
Update: YubiKey 5Ci is now fully supported.
Unfortunately, the AutoFill part is not allowed to communicate with hardware. This limitation is set by the system. As a result, YubiKey-protected databases can only be unlocked in the main app. We are exploring a possible workaround for this nuisance.