Yes I have seen some game server projects on raspberry pi:
In my humble opinion, it seems like it would be too slow to have an enjoyable experience.
What do you mean by too slow?
Yes, I heard that it's doable and pretty easy to make a game server with raspberry pi. I found this article with more details: https://www.toptal.com/raspberry-pi/how-to-turn-your-raspberry-pi-into-a-development-server