Freespace 2 Open, the games source was released in the early 2000’s under a … dubious licence that may or may not be legal to work with but no one really knows and it’s been more than 20 years so eh
Freespace 2 far and away my favourite space game, nothing else comes close to the tension of war that game setup so it’s great that you don’t have to run a .exe from the 90s to play the game. lots of mods for it now too thanks to this project