rel="canonical" introduced
Google, Yahoo and Microsoft introduced the canonical link element so publishers could consolidate duplicate URLs onto a preferred version.
What changed
The canonical link element gave site owners a way to tell search engines which of several near-identical URLs should be indexed and receive consolidated ranking signals. It was announced jointly by Google, Yahoo and Microsoft and immediately became a core technical SEO control for faceted navigation, tracking parameters and syndicated content. Google's guidance has always treated it as a hint rather than a directive.
Timing
Rollout began 12 February 2009. No completion date was announced. Rankings typically remain unstable for the whole rollout window, so measurements taken inside it are not a reliable read on the outcome.