Deleting a deck removes it and everything in it. This article covers how to delete a deck, what that actually takes with it, and why there's no way to undo it.
Deleting a deck
From a deck's detail screen, tap the ••• menu in the top-right corner, then 
Delete.
A sheet opens with a warning: Are you sure you want to delete this deck? This will permanently remove the deck and all of its flashcards. Tap 
Confirm to proceed.
A second, native confirmation follows: This action is destructive and cannot be undone. Are you sure you want to delete this deck and all of its flashcards? Tap Confirm again to actually delete the deck, or 
Cancel at either stage to back out.
Once confirmed, a "Deck deleted successfully" toast appears and you're taken back to your deck list.
What gets deleted
Deleting a deck cascades: it removes the deck and every flashcard in it, not just the deck itself. There's no option to delete a deck while keeping its cards.
This can't be undone
Deck deletion has no restore option anywhere in the app. Compare this to deleting a flashcard: a card deleted from Translation History stays visible there with a Deleted label you can tap to restore it (see Deleting a flashcard). A deleted deck has no equivalent. Once you confirm, the deck and its cards are gone for good.
Warning
There is no undo, no trash, and no recovery path for a deleted deck. Both confirmation steps exist because of this: think before you tap Confirm on the second one.
Effect on statistics
Your card and deck counts update to reflect the deletion. Past reviews of the deleted cards still count toward your overall review history and statistics; deleting a deck doesn't erase what you already studied, only what's left to study.