While you might want to cast me out for that pun, you might want to sit and hear what I have to say first
A typical Feather deck is full of instants and creatures that want to be buffed with them, which is exactly the kind of fresh deck that Boros needs
Like most Magic: The Gathering cards, the value of Halsin, Emerald Archdruid depends on a few consistent factors: its rarity, the printing and finish (regular or foil), its physical condition, and the set it was printed in (Commander Legends: Battle for Baldur's Gate)
Thats not to say it isnt great in other decks of three colors or more either
Its being warped by the eldritch influence on Innistrad, the plane which this card comes from, and its already taken a chunk out of the poor persons finger