devtrackers.gg
Seems to be datamined cardback? I'm willing to think it's a playtester/rioter exclusive or something tho
You don't have to datamine - go to your collection and filter to "LEGACY" and you can see it too.
Think of it as something like the Fist Bump ward skin in League or Fist Bump gun buddy in Valorant - a gift Rioters will be able to send to players they have a positive interaction with, but that's not quite ready yet.
Fist Bump ward skin in League
I have played league for 10 years and have never heard of or seen this