C'est intéressant, j'ai recherché la question, et j'ai trouvé deux exemples de cas où le code serveur avait été rendu public aussi:

World in conflict, un jeu Ubisoft justement

The game offers multiplayer functionality, supporting up to 16 players online or over LAN. In December 2015, Ubisoft shut down the official Massgate servers that supported multiplayer functions,[13] though the player community restored these functions in 2016, through an unaffiliated version of Massgate.[14] Ubisoft revived multiplayer through published open-source Massgate in 2017.[15][16][17][18]

https://en.wikipedia.org/wiki/World_in_Conflict

Duelyst

On January 10, 2023, the complete source code and assets for the game were uploaded to GitHub under the CC0 license.[4]

https://en.wikipedia.org/wiki/Duelyst

Après, il y a sans doute effectivement des spécificités techniques, mais toujours est-il que dans certains cas c'est possible, mais dans le milieu du jeu vidéo