At the minimum they'd need to store your name and address, probably birthdate too. It would just complicate the whole process of leaving Facebook.

so they’d delete everything but that. the rest of the leaving process would be the same (I recently did it myself, it wasn’t too bad, but more annoying than I’d like).

don’t get me wrong, i’m no fan of facebook and their data practices, but i don’t agree with what you said above.