I don't think this has been posted here before. The EC recently published an interpretation of the rules:
https://ec.europa.eu/transport/sites.../c20201830.pdf
It says (in 2.2):
It appears that various carriers are offering vouchers to passengers, who do not want to (or are not authorised to) travel any more as a result of the outbreak of Covid-19. Passengers can use these vouchers for another trip with the same carrier within a timeframe established by the carrier.
This situation has to be distinguished from the situation where the carrier cancels the journey and offers only a voucher instead of the choice between reimbursement and rerouting. If the carrier proposes a voucher, this offer cannot affect the passenger’s right to opt for reimbursement instead.
And in 3.2 it clarifies that the entire ticket has to be refunded even if only outbound or only return has been cancelled.
All of this of course only covers flight originating in EU (plus a couple of extra countries)