Based on research done in the last ten years , the only time it is necessary to feed a low protein diet is when your pet is uremic, which generally means BUN is over 80 mg/dL (equivalent to 28.6 mmol/L), creatinine is over 4.0 mg/dL (equivalent to 354 µmol/L), and the animal is showing symptoms such as vomiting, nausea,inappetence, ulcers and lethargy, which are caused by the build-up of nitrogen in the blood. Even then, feeding low protein will not extend life, but it will help the animal feel better.
If your pet has significant amounts of protein in the urine (urine protein:creatinine ratio above 1.0), then you may need to reduce protein moderately, enough to control the proteinuria, but a really low-protein diet is not considered beneficial, as it can lead to hypoalbuminemia (low albumin levels). It's best to feed at least 1 gram of protein per pound of body weight daily (the grams of protein must be calculated from a nutritional analysis, it is NOT the same as grams of meat).
If your dog or cat is not uremic, then the consensus seems to be to feed a moderate amount of very high quality protein. Eggs have the highest quality protein (although egg yolks are high in phosphorus), followed by meat (raw or cooked). The lowest quality protein comes from grains.