Blizzard has decided which version of World of Warcraft to use as the foundation for WOW Classic.
In a post on Battle.net, Blizzard said it decided on Patch 1.12: Drums of War “because it represents the most complete version of the classic experience”.
The classic period of World of Warcraft was two years long – and this period included a huge number of changes for the MMO as well as massive additions, such as Battlegrounds.
Source: Eurogamer Patch 1.12: Drums of War is the foundation for World of Warcraft Classic