My friends, i could compile an application written with Qt under 10.13.6 and run it on the newly upgraded ios 12.4.6 without any problems.
you need to export the following variables, which tell the linker to use sdk 12.1
####### Custom Variables
EXPORT_QMAKE_MAC_SDK = iphoneos
EXPORT_QMAKE_MAC_SDK_VERSION = 12.1
EXPORT_QMAKE_XCODE_DEVELOPER_PATH = /Applications/Xcode.app/Contents/Developer
EXPORT_VALID_ARCHS = arm64 x86_64
EXPORT_ACTIVE_ARCHS = $(filter $(EXPORT_VALID_ARCHS), $(ARCHS))
EXPORT_ARCH_ARGS = $(foreach arch, $(if $(EXPORT_ACTIVE_ARCHS), $(EXPORT_ACTIVE_ARCHS), $(EXPORT_VALID_ARCHS)), -arch $(arch))
EXPORT_QMAKE_XARCH_CFLAGS = $(EXPORT_QMAKE_XARCH_CFLAGS_arm64) $(EXPORT_QMAKE_XARCH_CFLAGS_x86_64)
EXPORT_QMAKE_XARCH_LFLAGS = $(EXPORT_QMAKE_XARCH_LFLAGS_arm64) $(EXPORT_QMAKE_XARCH_LFLAGS_x86_64)
EXPORT_QMAKE_XARCH_CFLAGS_arm64 = -Xarch_arm64 -miphoneos-version-min=11.0 -Xarch_arm64 -isysroot/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS12.1.sdk
EXPORT_QMAKE_XARCH_LFLAGS_arm64 = -Xarch_arm64 -miphoneos-version-min=11.0 -Xarch_arm64 -Wl,-syslibroot,/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS12.1.sdk
EXPORT_QMAKE_XARCH_CFLAGS_x86_64 = -Xarch_x86_64 -mios-simulator-version-min=11.0 -Xarch_x86_64 -isysroot/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator12.1.sdk
EXPORT_QMAKE_XARCH_LFLAGS_x86_64 = -Xarch_x86_64 -mios-simulator-version-min=11.0 -Xarch_x86_64 -Wl,-syslibroot,/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator12.1.sdk