Wasn't that WoW? I vaguely recall that a lot of the private servers worked off of a copied and / or decompiled version of their own server software for years, which is also why they never went further than the WotLK expansion. (the other part of that was people didn't want to, but that's another discussion)
AFAIK the WoW server code was never leaked. All private server code bases were developed by reverse engineering network traffic, game behaviour and client asset files.
E.g. the code for AzerothCore is fully available (and very easy to run on very low spec hardware)
https://github.com/azerothcore/azerothcore-wotlk