In the event of account closure, IBKR will mail a physical check with the proceeds if the account is held with IB LLC, maintains an address in the U.S. and has elected a Base Currency of USD. Note that the account holder also has to submit their account closure request online and complete all the prerequisite steps associated with closing an account before a check can be issued (including making certain that the address of record remains accurate).
Accounts which are not eligible to receive the proceeds from account closure via check will be prompted to add/confirm banking instructions at the point the closure request is submitted online.