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It still mentions it for me. On the "what's new" link there's a "Foundation" section where Metal and other performance enhancements are mentioned.
 
I still see it

The apps in iOS 9 now take advantage of Metal, making more efficient use of the CPU and GPU to deliver faster scrolling, smoother animation, and better overall performance. Email, messages, web pages, and PDFs render faster. And multitasking features on iPad feel fluid and natural.
 
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