The response to this question is dependent upon the context in which you are asking the question. In a laboratory situation, suspension stability is measured by dispersing a protein powder in ambient temperature water (10% w/w) and pouring the aqueous protein dispersion into a 100 ml graduated cylinder. One then observes how long it takes the first sediment to appear at the bottom of the cylinder as well as the total sediment volume over a specific time period (say, 4 hours). The protein that will remain in suspension the longest displays superior suspension stability and also the protein that settles out the least over a specific time period also displays superior suspension stability.
In real-life applications, however, suspension stability is much more commonly referred to as how well the protein powder remains suspended in water or other liquid after it has been thoroughly dispersed in the liquid. For example, in high protein ready-to-drink shakes, suspension stability is determined by how long the protein will remain suspended in the drink over the shelf life of the product. In cheese making, suspension stability would have more to do with how much settling out the protein powder evidences after being dispersed in milk.
Basically, when we talk about suspension stability, we talk about how well the protein stays suspended in cold to ambient temperature liquids prior to, or in the absence of, added heat. Once heat is applied to the dispersion, then suspension stability and solubility become one and the same.