martinibuster - 11:45 pm on May 23, 2013 (gmt 0)
I didn't know that "powered by" links had been targeted by Google, and for so long, too.
I heard it straight from Marissa Mayer at SES San Jose in 2003 or 2004 in a session about search crawlers. She said they would depreciate powered by footers from irrelevant sites. I raised my hand and asked her that specific question. Her answer was coming more from the point of relevance than the position of the link on the page. However the point is that was the first time anyone from Google had stated that this kind of link evaluation was happening and it was a bombshell revelation at the time. That was the first step in the evolution of the algorithm toward depreciating links like you described. There were more changes over the course of the next years in the way links are handled.
Later on Google introduced depreciation of links based on location within the web page. There already was a crude version of that in the original search engine, where content at the top of the page was deemed more important than content near the bottom, as well as matching title tags to anchor text etc. The idea of all these ways of treating links was to identify relevance but at Google was increasingly gamed the emphasis started tilting more toward identifying attempts to manipulate the rankings, figuring out how to separate real citations from those that are not. It's all been evolving year after year.