Is Protein good for muscle & strength?

Yes — the evidence for this is strong. A foundational meta-analysis and meta-regression established that protein supplementation meaningfully augments resistance-training-induced gains in muscle mass and strength in healthy adults, and this has been extensively replicated in older adults with sarcopenia specifically, including network meta-analyses comparing different protein types and dosing strategies. That's reflected in an evidence score of 82/100 for its role in muscle & strength, drawn from 10 studies, with an estimated impact on longevity rated substantial (68/100).