Not done installing all yet, but a pattern emerges(!): karsten ~ # qlist -o dev-qt | grep 5 | grep '\.a$' /usr/lib/libQt5UiTools.a /usr/lib/libQt5Bootstrap.a /usr/lib/libQt5QmlDevTools.a /usr/lib/libQt5PacketProtocol.a /usr/lib/libQt5QmlDebug.a /usr/lib/libQt5EventDispatcherSupport.a /usr/lib/libQt5AccessibilitySupport.a /usr/lib/libQt5FontDatabaseSupport.a /usr/lib/libQt5OpenGLExtensions.a /usr/lib/libQt5LinuxAccessibilitySupport.a /usr/lib/libQt5PlatformCompositorSupport.a /usr/lib/libQt5ServiceSupport.a /usr/lib/libQt5InputSupport.a /usr/lib/libQt5EglSupport.a /usr/lib/libQt5FbSupport.a /usr/lib/libQt5DeviceDiscoverySupport.a /usr/lib/libQt5EdidSupport.a /usr/lib/libQt5ThemeSupport.a
All of these *.a libraries have no corresponding *.so libraries. The same behavior is in older versions of Qt 5 (e.g. 5.7 or 5.9). At least some of these libraries are actually used by other packages (e.g. kde-frameworks/kjsembed:5 and kde-frameworks/kross:5 use libQt5UiTools.a).
This is expected. These libraries are static only.