I think there are a number of factors that play into this discussion and there is not a "one size fits all" answer. There is "plus P" and "plus P". The SAAMI standard for plus P when the earlier versions of the J-frames were produced was 18,500 psi for plus P. Many loads today are loaded to 20,000 psi. If S&W was leery of the 18,500 psi loads, they would look less favorably on the 20,000 psi loads. Furthermore, few people have doubted over the years that a steel-framed J-frame could handle at least a limited amount of plus P loads. The question, though, becomes more problematic when dealing with the earlier airweights with their aluminum frame and, more particularly, their aluminum topstrap. Plus P under the old standard was 8.8% over standard pressure. A 20,000 psi load is a little above 17.6% over standard pressure. Not insignificant, perhaps, for an aluminum topstrap designed for 17,000 psi. When Smith and Wesson "magnumized" the J-frames, incorporating features developed while designing the .357 J-frames, they subtly changed some features on the frame, .e.g. radiusing the square-cut corners on the cylinder window in order that the frame could better stand the forces involved without developing tiny cracks and, I believe, they subtly increased the thickness of the topstrap (could be wrong on that). No one questions the cylinder being strong enough to contain the pressures. Many question the aluminum topstrap being able to handle a steady diet of higher-pressure rounds without stretching. Topstrap stretching will, either quickly or slowly, ruin a perfectly good gun because it will eventually result in light strikes and misfires. Will an older airweight blow up with plus P? No, it won't. Will it have its service life significantly reduced? Many think so, including Smith and Wesson, and I agree with them. Anyone can shoot whatever they want in their own gun in the final analysis. Just my opinion and I've been wrong before, but there it is.
By the way, I had an early 642-1 not marked plus P and a call to Smith and Wesson resulted in them telling me that any of the 642s with a "-1" were plus P rated, including my 642-1. This was about the time that Taurus had a very large advertising campaign touting the +P capability of their small framed .38 revolver, the Model 85. Methinks, that campaign influenced Smith and Wesson in their rating decision.