UKNOWN won the GoT bet?
Bran should be winner as he's ruling Westeros.
but the throne got destroyed

Who will win the Iron Throne and rule Westeros at the end of Game of Thrones Season 8?
In our opinion, you can make valid arguments that both Bran Stark and Unknown are the outcomes. We plan to share the prize pool equally to those bets.
Who will win the Iron Throne and rule Westeros at the end of Game of Thrones Season 8?
Bran is ruling Westeros, he wouldn't be able to sit on the Iron Throne anyway. He's ruling from his Iron Wheelchair.
All other bookmakers list him as the winner.
Other bookmakers phrased the question differently. Brian didn't win the Iron Throne but does rule Westeros at the end of Game of Thrones Season 8.
In case there is no winner, or somebody other than the characters listed wins the Iron Throne, "Unknown" will win the bet.
That is another outcome, there is no clear winner so Unkown is also a winner. That's why we think that paying out on both is the best decision.