Not necessarily. For a game like this that only functions online, you could presumably determine all the possible server calls and point them to a server you own. You could do this purely via clever network settings without modifying the game at all. If you could do that, the game would run fine and you could even use the original authentication server to ensure the user holds a valid license.

At that point, you "just" need to implement and run a server for the game. This also doesn't involve modifying the game, but could run afoul of potential laws against reverse engineering if not done in a clean room manner (I'm not a lawyer so there could be other things too since unfortunately US law tends to not favor the end user).

Regardless of any of that, it always feels silly to me when companies fight tooth-and-nail against people not only performing free work and hosting for a dead game but ALSO trying to ensure people actually own the game before playing on their private server. Of course they could just use 🏴‍☠️ versions and black-hole the authentication server. All the company does by withdrawing licenses is ensure they have to skip authentication so the company loses out.