It's a random event, it's active as long as you haven't selected the game option called "No Random Events".Does the 'discovered treasure in city ruins' mechanic exist on all mapscripts?
And are there any requirements for it to come in play? (eg a razed city should have at least 2 population, or some buildings, or exist for more than 1000 years etc)
I'm sure I've seen it in work in c2c_pangea or c2c_continents (not sure which), but I spent a game in c2c_world with 2 city ruins and nothing came up till industrial era where the supposed treasure is practically useless