Ah, I see it; I had looked there but not noticed it hidden

amongst the "small print"

.
It was introduced in the November 2011 release. But, I believe, that it relies on the presence of the Do Not Track Header at the site (I think Firefox is the same), if it exists and for which there is no standard. Consequently Google's weasel words about its (non)effectiveness.
I believe that IE also now recognises the DNT header (since IE10??), if it exists, but it certainly continues to use its original user downloadable and controlled lists of sites, to which tracking information is actually blocked, as well.