We’re in the process of temporarily disabling The Lich King in Battlegrounds due to an unforeseen issue. We’re expecting to get the issue fixed and have The Lich King reenabled in our next major patch. Thanks for your understanding.
Update: It’s going to take a little longer than originally anticipated to disable the Lord of the Scourge. We expect The Lich King to no longer appear in Battlegrounds matches within the next ~24 hours.