Using an unopened bottle of soy sauce that is five years old is generally safe from a microbiological standpoint if it has been stored properly away from direct sunlight and heat, because high sodium content inhibits bacterial spoilage. However, the quality, flavor complexity, and aroma may have degraded significantly due to prolonged oxidation. If the bottle has been opened and exposed to air over those years, or shows signs of unusual discoloration, mold, or off-odors, it should be discarded immediately.