Actually, it seems that all vouchers issued between March 2019 (i.e. one year prior to the Corona outbreak) & December 2020 have had their validity extended to the end of 2021; so it doesn't matter what reasons a voucher was issued for, or what expiry date or conditions were associated with any voucher issued between 1 March 2019 and 31 December 2020 inclusive - all such vouchers are now valid until 31 December 2021.

https://www.klm.nl/en/information/fa...ravel-vouchers