Where is libstdc++.6.dylib in xcode10 beta?
Can't compile ios project.
How add libstdc++.6.dylib in iOS12?
Correct. It is also not present in the iOS Simulator runtime. If you want to hack around this, you will need to copy the libstdc++.*dylib from the iOS 11.4 simulator runtime to the iOS 12.0 simulator runtime in addition to copying the libstdc++.tbd between the SDKs.
Note that at some point the store will stop accepting apps linked against libstdc++, so it behouves you to actually update your project to use libc++ (which has been preferred for the past ~5 years or so).