[Thaxll]

WoW classic has been fully reversed engineered way before Turtle WoW released, the thing they did better is to extend the game with a lot of content, but the core wow experience has been emulated for a long time.

20 years ago I was already using Mangos to play wow classic. See: https://news.ycombinator.com/user?id=marenkay

[Salgat]

Not only content, but countless bug fixes and new features (such as the guild bank). There's also a related project (SuperWoW) that injected a dll to fix and extend the client (which is still Blizzard's old original client).