No, cats should not eat soy sauce because of its ingredients.
Originating from China, soy sauce is a liquid condiment made out of soybeans, wheat, salt, and a fermenting agent. Soybeans may be a plant-based protein that cats can eat occasionally, but some cats are known to be allergic or intolerant to it. If she is allergic or intolerant to soy, she may display the following: vomiting; diarrhea; flatulence; frequent scratching; hair loss; red, inflamed skin; chronic ear problems; and coughing, wheezing and sneezing. The same goes for wheat; some cats may develop an allergy or intolerance to it.
The high sodium content in soy sauce is another factor of concern. Concentrated amounts of salt can cause sodium ion poisoning, which can manifest as: vomiting; diarrhea; appetite decrease; lethargy; incoordination; excessive thirst or urination; tremors; seizures; and coma. In extreme cases, your cat may even end up with organ damage.
What to do if your cat accidentally eats soy sauce: A lick of soy sauce won’t do your pet cat any harm. However, if she ingested a considerable amount, contact your veterinarian immediately and observe her for any of the symptoms stated above. Keep an eye on her for the next 24 hours. Symptoms of sodium ion poisoning may be witnessed within three hours of ingestion.
In summary: Soy sauce will not provide any nutritional value to your cat. Because of its potentially harmful ingredients, the light brown, flavour-packed liquid should be kept away from your cat’s reach at all times.