(Given Google's near-monopoly of the market, Microsoft and DDG have some amusing competitive synergy going on, don't they. DDG can criticize Google all they please for retaining user data because DDG doesn't and isn't in a position to benefit from it. Microsoft, which certainly is in a position to benefit from it, doesn't need to worry about Google calling them on it because Google is the only search engine that can actually lose market share over the issue.)
It could be something a registered user could set from a browser toggle, and DuckDuckGo is a very good project, or course. My point was: data portability and user control are within Google's long term interest, not being evasive about their data cache.