WoW server emulators have been in development for over a decade, they’re public projects on GitHub and released under opensource licenses, all private servers use those emulators, none of them has original code, it’s never been stolen.
You can compile them for Linux (as well as Windows), how well they work depends on the version, up to WoTLK they’re fine, from Cata on they’re quite bad.
You will need the original clients that are Windows only but they work flawlessly with WINE.