Old Cruisers is a generic term for pre-war exploration and multi-role ships drafted into the Fleet's line of battle during the Romulan War.
When new Earth Force batle groups appeared on the scene, ther Old Cruisers were considered redundant and ill fitting in the planned fleet combat modes.
However, during the mid point of the war, when all the Allied fleets were learning how to fight Romulan Incursions, the Old Cruisers multi role capacity and aadaptability proved to be assets.
After the Romulan war, surviving Horizon and Bonaventure class ships proved popular among crews. The resumed their pre-war role of explorers and patrolers, until superceded by newer ship types.