It is not a new policy.An S-Stock train I'm currently on is only releasing doors, rather than opening them. Is that a new policy?
Is it controlled by a setting, or selected each time the Doors Open buttons are pressed?
Was this underground or outside? It might have benefits in cold weather at lesser used stations.
Down to driver's discretion. There is no hard and fast rule about door control but generally kept under driver's control.
As it is bl**dy freezing at the moment, the driver is thinking of you lot and doing his best to keep you warm!