Last modified: 2012-12-12 15:56:10 UTC
Where did you find the occurrence of this message? Judging from "articlefeedback-" prefix, I'd say it's part of the original AFT extension, but could not find the label searching that extension either
There was a time whe someone decided that it might be a good idea to blanket import all ArticleFeedback v4 messages in ArticleFeedback v5. It's not unthinkable that some cleanup was done since the original report in May 2011.
IIRC there was a separate section a bit down in the English section of the i18n file. There may still be unused messages there, maybe not.