В комментариях к офф посту в блоге было сказано:
Sam, the fact that this only works on Windows 8.1/Windows Server 2012 R2 is in no way a statement about platforms that it will eventually support. Those targets (which are identical from the JIT perspective) were just the lowest cost of entry for a not-ready-for-prime-time JIT compiler. Honestly, I have no idea where this is likely to ship first, so we just wanted to figure out how to ship it for customers to kick the tires.
Встречайте RyuJIT: новый JIT-компилятор для .NET