A bit of a late comment to this thread but having discussed the drop rate with my clan, we think there's a bug definitely. None of us have ever had them drop (6 of us x numerous boot exotic engrams) and I myself never bought them from Xur as I didn't have rise of iron until after he'd sold them. We have seen one person wearing them during our time playing. One member of my clan has every exotic bar these boots and is incredibly frustrated by it. Yes it may be a case of RNG but underlying that, I think they haven't removed the 'fix' they made when the perk could be transferred and it seems Bungie can't see how much this annoys players who want to complete their collection.