A new-to-MTG Arena mechanic, not exclusive to the Alchemy Horizons: Baldur’s Gate set, boon is a way to represent delayed triggers, similar to an emblem, but once they trigger, they disappear.
Boon is a digital-only, Magic: the Gathering Arena exclusive keyword that was introduced in Alchemy: Innistrad, but officially made a keyword in Alchemy Horizons: Baldur's Gate. This is a way to create triggers that can activate later under a particular restriction to help increase design space.
Unlike Specialize and Double Team, Boon itself doesn’t have a rigid effect beyond it’s a delayed trigger that something in the future can activate. The power of a Boon, much like any activated or triggered ability, will be contextual on what card it is on and how it can impact the game state. For context, cards like Tenacious Pup and Molten Impact are both boon cards and some of the best in Alchemy, so considering that Boons tend to be a bit more restrictive by nature, the effects have been relatively strong.
As of June 24, 2022 there are 4 cards in MTG Arena using the Boon keyword and 4 cards spoiled in Alchemy Horizons: Baldur's Gate referencing Boon. As this is a digital-only ability, cards have been designed only for the digital MTG Arena formats such as Alchemy or Historic. Here are the cards with Boon that we see so far.
3 cards with Boon were introduced with Alchemy: Innistrad, the first digital-only set for the brand new Alchemy format. These cards subsequently also became legal in Historic.
1 card with Boon were added in Alchemy: Kamigawa.