The release notes for Xcode 10 beta 2 say:
Building with libstdc++ was deprecated with Xcode 8 and is not supported in Xcode 10 when targeting iOS. C++ projects must now migrate to libc++ and are recommended to set a deployment target of iOS 7 or later.
We're currently supporting iOS 6, so the question is what will happen if we switch to libc++ instead of libstdc++. From Apple's wording (setting the deployment target to iOS 7 is only recommended, not required) it seems iOS 6 has libc++, but I've not been able to find any documentation about this.
From the wording it also seems that there might be problems with libc++ on iOS 6, but again, no other documentation, so the second question is what kind of problems could that be? Is there anyone who tried libc++ on iOS 6 years ago who would know/remember anything?
libc++ was introduced with iOS 5.0: http://deltaorange.com/2012/11/21/c-compatibility-on-ios-less-5/