Am I obligated to pay Zakat on a plot of land that the state benefited from for 30 years, and has now decided to compensate me financially for it in exchange for relinquishing ownership?

If the land was not rented or sold to the state, then there is no Zakat due on it for the past period. Zakat becomes obligatory on land if it is for trade, or on its rent if it reaches the nisab and a full year has passed over it, or on its deferred price (Zakat on debt). If the money is not reliably attainable, then there is no Zakat on it until it is received and a full year passes over it. Zakat becomes obligatory on the compensation amount if it is received and a full year passes over it from the day of its receipt.