https://www.pcgamer.com/world-of-warcraft-classic-will-be-built-on-the-112-drums-of-war-patch/

Blizzard said last year that making World of Warcraft Classic is a "deceptively tough" job. It expanded upon that theme today with a development update digging down into the early stages of the process, beginning with which version of the game it wanted to build Classic on. 

"As many of you have noted, the classic period was two years long and full of changes. Core features like Battlegrounds were introduced in patches after WoW’s original launch, and class design similarly changed over time," Blizzard wrote. "After careful consideration, we decided on Patch 1.12: Drums of War as our foundation, because it represents the most complete version of the classic experience."