It’s just shiny black card.
I must be the only saddo that keeps the nicer card holders then! I’ve still got both of the different Starling card holder designs and a Curve one I think (the one that’s like the Starling one, not that super fancy one)! Or maybe it was Revolut… If so, I’ll be finding it and burning it ceremoniously!
I probably have my original Monzo beta one somewhere. I know I’ve still got the card with that nice 3D printed holder someone on here was making!