Go to Magic.Wizards.com/Formats for a complete list of formats and their permitted card sets and banned lists
Here's how the combo works and why it's so strong
It's still a potent Pokmon because it does 300 damage for just two Energy, and in Expanded form, it's a formidable Pokmon to discard strategies with Gengar from Sword/Shield: Lost Origin using Battle Compress
Sadly, control decks are some of the worst decks for flipping battles since they usually don't play many creatures, which means the Serra Angel backside is unlikely to show up very often
On top of finding standard foil versions of cards, you might also chance upon Lorcanas incredibly rare Enchanted cards - special variants that feature borderless designs, brand new artwork and a holographic rainbow effect
It really simplifies things, being able to locate particular cards in your collection with the click of a couple buttons