That's a good question. I wonder what the number would be for Stellaris? With myself, for example, I don't think I've played any other games with mods. I would certainly be in the 80% for Civ5, while on the other hand I can't see playing Stellaris without them.With popular games, even 4X ones, a relatively small number of userbase uses mods. According to Steam achievement stats, for instance, with Civ5 it's about 20%.