Mvq
2016 TC Champion
I know for a fact that Blizzard can't track old quest rewards that were deleted more than ~three months ago. (They also can't track whether the quest ever was completed or not, if it's been removed.)
That's an odd claim. They can track if you've completed a quest simply with the API. Heck, you can even check it yourself. Just write this in-game:
/script IsQuestFlaggedCompleted(questID)