According to code retrieved from Pokemon Sword and Shield, there SHOULD be a 1/16 chance that a Shiny Pokemon would be a Square Sparkle Shiny Pokemon – but right now odds are a bit broken!

The front face of the test print copy of Blastoise from the Pokémon TCG that's currently up for auction at Heritage Auctions. This card uses the art also used on the box for Pokémon Blue Version.