What, part of the Agreement (whichever Agreement it was), and a
crucially important part of it at that, covering the very basic issue of
whether there are tariffs on sales between the parties' territories, has
simply been repealed without ceremony?

repealed and replaced

"it has undergone a final process of legal revision in accordance with
Article FINPROV.9 (now Article 780), and the version published below
represents the version agreed in December, prior to this process. The
substance of the Agreement has not changed; however, typographical and
other errors have been corrected and the Articles have been renumbered
from Article 1 €“ 783"

https://www.gov.uk/government/publications/ukeu-and-eaec-trade-and-cooperation-agreement-ts-no82021