As "it represents the most complete version of the classic experience" the development team behind World of Warcraft Classic decided to use the 1.12 update as the foundation for the upcoming old school version of the game. A decision that was revealed in a recent dev blog.
This version, which at the time introduced battlefields such as the Warsong Gulch or the Alterac Valley will not include certain more modern features, such as Achievements or Transmogrification, for example. The development team also confessed they face some difficulties in the development of the game, because of the compatibility problems between our modern version of the game and the version they're trying to replicate.
"All the work we're doing will ultimately allow us to recreate an authentic classic experience on a platform that is much more optimized and stable, helping us avoid latency and stability issues. Additional improvements will include modern anti-cheat/botting detection, customer service and Battle.net integration, and similar conveniences that do not affect the core gameplay experience."