Today, The Qt Company announced the release of Qt 5.12 LTS. This release brings with it support for Xcode 10 for the first time. ArcGIS Runtime SDK for Qt 100.4 was built with Qt version 5.9, which for iOS builds, utilizes Xcode 9.
What does this mean?
If you develop iOS apps with ArcGIS Runtime SDK for Qt 100.4, you should continue to use Xcode 9. We have performed some ad-hoc testing with Xcode 10, and have not found any new issues, but it is possible that there are some binary compatibility issues, and we therefore do not recommend this. Starting in March 2019, Apple's App Store will require that all submission be built with Xcode 10. We recommend pushing any required updates before this date and waiting for the 100.5 release of ArcGIS Runtime, which will support Xcode 10. We are planning on releasing 100.5 near the end of Q1 2019, so we hope this will cause minimal issues.