When updating Xcode to version 10.0 it become impossible to load a project. Project loading always fail with error :

Project ERROR: This mkspec requires Xcode 4.3 or later

It look like the check does a string compare so he find 10.0 lower than 4.3

I found how it is checked in the file :

Qt_folder/5.9.6/Src/qtbase/mkspecs/features/uikit/default_pre.prf 

which is the same for each platform on each version:

# Check for supported Xcode versions
lessThan(QMAKE_XCODE_VERSION, "4.3"):
error("This mkspec requires Xcode 4.3 or later")

ios:shared:lessThan(QMAKE_IOS_DEPLOYMENT_TARGET, "8.0"):
QMAKE_IOS_DEPLOYMENT_TARGET = 8.0