Do I have to accept a voucher if I think I'm owed a refund?
The Statement on Vouchers suggests what could be an appropriate approach in extraordinary circumstances, but doesn't affect airlines' obligations or passengers' rights.
Some airline tariffs might not provide for a refund and others might include force majeure exceptions to refund provisions.
If you think that you're entitled to a refund for a flight that was cancelled for reasons related to the COVID-19 pandemic and you don't want to accept a voucher, you can ask the airline for a refund.