API homepage: https://developers.google.com/chart/image/
The April 20, 2012 announcement: http://googledevelopers.blogspot.com/2012/04/changes-to-deprecation-policies-and-api.html
I don't know if they ever announced an official date for the deprecation of the charts API, but I had always assumed it would be 3 years from the announcement date...which may have just been a mistaken assumption that Google APIs all previously were under the 3-year policy?
In any case, almost all the other APIs that were announced as retired in that Apr. 20, 2012 post are dead (Moderator, Portable Contacts, Feedburner) or superseded...the ClientLogin does mention an exact shut down date of Apr. 20, 2015.
The static image charts API page was most recently updated in January 2015. The deprecation note simply says "we officially deprecated the static Google Image Charts way back in 2012. This gives us the right to turn it off without notice, although we have no plans to do so."