I've asked around among friends and also Reddit but we've not reached a consensus: How does Companion work after sideboarding?
I can find the following relevant rule snippets.
Your companion's deckbuilding rule applies only to your starting deck, which is the deck you begin play with each game. It won't consider any cards in your sideboard, so those cards don't have to follow the deck-building rule.
(Source: Ikoria: Lair of Behemoths mechanics)
In Magic: The Gathering, a match is a multiplayer game or a two-player series of games (usually best-two-of-three) played in a tournament.
(Source: https://mtg.gamepedia.com/Match, referencing the official rules)
I cannot find the concept of a "starting deck" being mentioned anywhere in the rules.
Reading the above snippets makes me believe that in games following sideboarding a Companion card cannot be chosen as the Companion for that game if any of the cards in the new deck obtained after sideboarding violate the Companion requirements.
Is this correct?