Cressington is not the only station which requires SDO, but is unique in that it requires an entire unit to be locked out rather than just one door.
For those unfamiliar with the station: Cressington sits in a cutting with road bridges at both ends of the platforms. When the station was reopened and upgraded for Merseyrail in 1978, the platforms were extended into the cutting under the road bridge at the Liverpool end. The platform extensions are very narrow and required special dispensation for construction, even by the more relaxed standards of the 1970s. For as long as 507s and 508s ruled the roost, grandfather rights applied. The arrival of the 777s required an alternative solution. The "normal" width platform can only accommodate a 4-car unit.
But yes, it is ridiculous that we are still waiting on a solution. It is nearly six years since the first 777 was delivered!