Right after I posted my last message, I searched my email again and found an e-mail from Tap in December that looked liked a promotion which I ignored (the subject was "95 destinations with TAP Voucher"). There I found:

- They cancelled the previous voucher and created a new one with extra 20€ there (~5% of the original voucher amount), same expiration date. When I first issued my voucher in April 2020, they were not giving any extra amount, which they seem to be doing now... so they decided to be nice with old voucher owners (and pay some interest on my money lending).
- There is a link where I can go to: (a) extend the validity of the voucher by 6 months in case the original voucher expiration date was before 31DEC2021 (not my case), or (b) get the refund on the voucher on my bank account.

With that, I'll convert the voucher to cash, but obviously without the 5% they deposited recently.

In case anyone is interested, the link is https://www.flytap.com/pt-pt/voucher-options?airbank= and at the end of the URL (after the "=" sign) you should add the voucher code (in my case, "VCHCV...."). Then you confirm your e-mail address associated with the voucher and then the 2 options show up.