I was provided with a voucher by BA in lieu of a deposit I paid for a flight and hotel package with the company that was cancelled in 2020 due to Covid. The voucher remained unused and I received an e-mail from BA this morning informing me that as it hadn't be used within the specified timeframe they would refund the money to my payment card.
BA are evidently being proactive in terms of refunding passengers with expired vouchers so if you have any due to expire then expect a refund without having to apply for one.