Looks like Blizzard added an automatic buff for the next week on Anniversary realms, only works in the open world though not in instances.
Blizzard Entertainment
This is to celebrate WoW’s 21st Anniversary on Anniversary realms.
The buff will be available for one week, and will end at 10:00 a.m. PST on Monday, November 24.
Source: Unrelenting Rallying Cry of the Dragonslayer Buff Appears on Anniversary Realms - Warcraft Tavern
