Implementing a WoW classic server is actually fairly easy. The game client comes with the entire engine, art, music, and quest content. The server is basically a fancy IRC server, taking client events and rebroadcasting them to other clients.

Even many of the events are implied, like how regular attacks continue at a fixed frequency once started, so other clients only need to know when the player started attacking and whether they are still in range, and player run speed is a constant so a player running in a straight line doesn't generate additional events.

I even suspect the dice rolls are coming from a shared RNG that each client maintains independently, but haven't researched it.

This is how WoW classic was playable over a 33K modem.