Your sideboard may now only contain cards printed in the set of your deck.
More precisely, here is the new sideboard rules: “Your sideboard may only contain cards printed in the set of your deck, in any number of copies (e.g.: if you’re playing a Throne of Eldraine deck, it may contain 8 Oko, Thief of Crowns). However, your deck must still be compliant with the Primordial deck-building rules after swapping (2 mythic rare / rare, etc.). This is meant to be for wish players (e.g.: Fae of Wishes).”
The previous rule (that allowed you to have cards from different sets in your sideboard) aimed to enhance the concept of “cards you own from outside the game”. However, this seemed to do more harm than good to the popularity of the format.