Where are all my cards/decks after migration? ?

To be more accurate:

• HGSS-era cards are gone.
• Cards over the number of copies you can play in a deck are gone.
• Decklists are gone.
• Only cards from Sun and Moon-on are functional.
• BW and XY cards are in the game and transferred, but you'll need to set your filters to show all cards to see them. Expanded won't show them, and they aren't usable. They're supposed to be made playable eventually, but don't hold your breath.