The way I understand it is that a request has been made repeatedly by the Galician government to the Santiago archbishop to ask the Pope to grant an extension. Which the Pope is expected to grant if asked. However, the archbishop hasn't asked yet and will apparently do so only
if and when the time has come where such circumstances arise, or something like this.
Again, the way I understand it, the reason are not any multitudes that are expected to come in 2021 but rather the opposite: a chance to make up for lost business and the fear that not enough visitors will come in 2021 as there may be still Covid-19 related restrictions in place or considerably fewer visitors because of their fear of travelling to Spain. The archbishop is apparently determined to declare the start of the Holy Year as planned on 31 December 2020. A possible extension may already end on the saint's feast day in July 2022.
Lots of cultural events and promotion are associated with the Holy Year 2021 and the aim of the Galician government is to spread the tourist manna all over Galicia, not only along any
Camino de Santiago.
A recent article, dated 4 June 2020, is here:
In
Spanish
In
English (translation by Google Translate)