Are there any? With dedicated communities behind titles like FreeCiv, and the almost “roguelike” character these games can have, I’d expect that it could happen. It’s also heavy on mechanics and light on presentation, another thing open source games tend to be better at on average.
0 A.D. comes to mind. https://play0ad.com/
While it is a fun game, I think it’s really more of a traditional RTS and not really grand strategy.