So im 8 years into playing and im still yet to find the Cosmic wings. I realise its the rarest item in game however i think its time to make them a bit easier to find with D4 coming. For me finding the wings would be a nice completion of the game.
In S27 i found zero rainbow 
Goblins. I also found 1000+ Holiday Gifts and nothing.
I am suggesting perhaps the drop rate increased slightly or even the Rainbow Goblin spawn rate to be increased to give more chance to find them.
If you played for such a long time, this sounds very suspicious. Back when those wings were released, the communities for farming it were always packed. There were offers for rainbow goblins almost every minute.
I haven’t done the math, but I think Wirt’s Original Leg is rarer and you can’t even use the transmog in seasons.
Sadly, I found under 100 and got the Cosmic Wings twice, the first time being left on the ground and the second, sadly, being picked up because I was beam happy and grabbed everything.
I really wanted to get them the usual way, if I got them at all.
But, seeing as I don’t use wings, they are rare and 100% useless.