Why does Apple clang disallow C++11 thread_local when 'official' clang supports it

The clang compiler included with Xcode 8 and later supports the C++11 thread_local keyword. This functionality was added to the Xcode 8 beta as discussed in the WWDC 2016 video "What's New in LLVM", beginning at the 5:50 mark. (external transcript)

The sample program listed in the question compiles and runs with Xcode 8 GM under OS X 10.11.6 and produces the intended output. It has subsequently been re-tested with Xcode 9.3 under macOS 10.13.4, and with Xcode 10.2.1 under macOS 10.14.4, and continues to behave as intended.

Regarding iOS, I found by experimentation that thread_local is supported for iOS 9 and later, but not for iOS 8.4 or earlier.

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For Xcode 7.x and earlier, here is an answer from 2014 from an Apple engineer on the old Apple Developer Forum (no longer accessible):

We don't support the thread_local implementation from the open-source Clang because we believe we can provide a higher-performance implementation for our platforms using various features in the dynamic linker. Such an implementation would be ABI-incompatible with the implementation in the open-source Clang, so we won't support thread_local until we get an implementation we can live with for the foreseeable future.

A subsequent post confirms that thread_local is still not supported in Xcode 6.3.