Brann Bronzebeard appears to have been rolled back to level 15 for any Warband that had leveled him higher.
Update: The rollback did not appear to nerf levels of his talents. A formerly level 60 Brann appears to be a level 15 Brann that has level 60 combat capabilities.
Players have report that Brann Bronzebeard has started losing levels including a Reddit Thread, players on WoW Armory who had a level 60 Brann and other reports through Discord.
12 Days ago, Blizzard hotfixed Brann to stop gaining experience at level 15.
By this point, many raiders in the top guilds had worked their way to a capped, or close to a capped Brann at level 60. There were multiple ways to do this quickly including multiboxing and also buying/selling Brann Experience on the Auction House.
Brann levels will be important starting next week as Delves increase to Tier 8 and will become much harder. Brann gains 5% healing and damage for every level past level 10, making a level 60 Brann much more powerful than a level 15 one.
It seems that in the interest of fairness, hotfixes have now rolled back any player with a Brann Bronzebeard over level 15 down to level 15, starting all players at the same place.
That is… if the hotfix actually nerfed the Explorer's Medical Journal on Brann.