They put deprecation warnings in the headers starting with 10.7, and I'm pretty sure there was a WWDC talk (hopefully still available) mentioning it.
These two links roughly explain the issues/reasoning:
http://rentzsch.tumblr.com/post/33696323211/wherein-i-write-...
http://ludovicrousseau.blogspot.com/2011/08/mac-os-x-lion-an...