This poorly titled thread seems to be where people are voting for this feature.
Apparently it's coming soon. They say it's available for Apple. Why would they start with Apple and not the OSes that is used by far more people?
I concur. I am a software developer and understand the need to get paid, but we're talking about an electronic grocery list here. I thought Remember the Milk Pro's $25 (USD) was ridiculous. Wunderlist Pro's price makes that seem reasonable. Get your feature set straightened out, lower the price and make up for it in volume (you're way better than RTM, BTW).
1,003 votesAdminWunderlist (Admin, Wunderlist) responded
The Wunderlist 3 iOS and Mac apps already find completed tasks when doing a search
Why do you hate the completed items list so much? This, and more importantly, the fact that you can't sort it (http://wunderlist.uservoice.com/forums/136230-wunderlist-2-feature-requests/suggestions/3683198-option-sort-completed-items-separate-option-to-), make Wunderlist much less "wunderful".
I can't fathom why they chose not to sort completed items when they implemented sort. It boggles the mind. The shopping list example (you continually move items out of the completed list into the uncompleted list when you need them again) is exactly why this is so necessary! As a matter of fact, sorting the uncompleted tasks is way less important that sorting completed tasks (perhaps completely unnecessary!). I like to order my grocery list according to the store layout (e.g., produce, then dairy, then meats, etc.). I don't need my uncompleted list sorted, but I do need my completed list sorted!