Rocket League 20xx Black Market Decal was added into the game a long time ago in the Turbo crate. Even though this decal may have been out a while, the 20xx Rocket League decal is one of the most popular black market decals that has maintained its value for a very long time meaning you won't lose value once purchased.
Is 20xx Paintable? No.
Is 20xx Certifiable? Yes.