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It was incomplete and requires Mac OS X 10.10, making it not a marketable feature until 10.10 was released.
 
My real question is, why do we have to download iOS 8.1 to get SMS relay?

They could just have said, "You can now make calls and texts from your iPad. And, when OS X Yosemite comes out, you'll be able to do the same from your Mac."
 
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