There has been a lot of confusion over this, including by IMT staff, since the new law came in force in February of this year, not helped by IMT leaving on their website the document contained in the original post which is out of date, the current document is well hidden on their website and only in Portuguese.
The current up to date document
A non EU driving licence (UK one after 31/12/20) is only valid as a licence to drive for 90 days from the date of issue of your resident's permit/certificate (Article 125 of the Código da Estrada).
However whilst it is no longer permits you to drive from day 91, it can still be exchanged for a Portuguese licence without the need to take a Portuguese driving test for up to 2 years from the date of issue of the resident's permit/certificate (Artlcle 128 of the Código da Estrada).
The IMT online portal explains it simply but I thought I would explain why people have got confused because two different legal articles are in play, and only one of them was changed to 2 years (done so to bring such licences into parity with expired Portuguese driving licences).
IMT online portal
Only if you have started the exchange process and have been issued the temporary driving permit, or already have a Portuguese driving licence law 87-A/2020 has extended the expiry date as Peggy has pointed out above in her second paragraph.
The blanket extension in those circumstances is until 31/3/21 and is further extended to 31/10/21 if an appointment for renewal has been scheduled. It only applies to Portuguese driving licences and temporary driving permits issued by IMT when the exchange process is started.
My apologies for going on a bit, but there are a lot of Brits (and Americans) being incorrectly advised that Covid means they now have 2 years before they need to exchange their licence. If they became a registered resident before 2/10/20 they are going to find themselves unlicensed from 01/01/21 if they follow that incorrect advice.