Yes. Contest Flashback works on any completed DraftKings contest whether you were in it or not.
Open the Contest tab on a past slate and click the contest name. You’ll be able to see a list of both all of the lineups and all of the unique players that entered that contest, along with the ROI of those individuals lineups and players.
You cannot, however, simulate what it would have looked like IF you had played specific lineups into a contest in flashback. We are hoping to add this kind of feature in the future.