Foster formed a formidable partnership at the centre of the Iron's defence alongside captain Andy Crosby, impressing enough to be voted the club's player of the season by fans.
No other out-of-contract players have been offered deals to stay, with the departures of Steve Torpey, Robbie Foy, Ramon Calliste and Ashley Allanson already announced.
Neil MacKenzie and Daniel McBreen are under contract, but have been told they have no future at the club and are free to leave.