The IRS will deposit it to the old bank account. The bank will reject it because that is wrong. Then the IRS will send a paper check or a debit card by mail. Watch your mailbox for the payment. It will be in a white envelope with IRS on the return address.
In a worst-case scenario where the IRS is neither able to direct deposit nor send a check in the mail, then the stimulus payment may be claimed as additional refund on the 2020 tax return.
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