Wing Covers still drop From Normal Coffers?
I don’t think you could ever get them from the normal coffers.
They have only ever been lootable items, never from coffers
I also never saw a Holographic Dragon Wing Cover drop from anything other than a mob. It doesn’t drop from the Coffers/Chests.
There is a small chance to get one from any enemy in the world or even from other players in WvW as a random drop.
They will stop dropping tomorrow when the new update hits, so make sure to get it now while it’s cheap. As of now you can still get it for under 10 Gold on the TP (which it would cost at the merchant if Zhaitaffy was 1 Copper – the lowest possible price for that). But right now Zhaitaffy is getting more and more expensive, so that is not an option.