Data from 49 studies with 1863 participants showed that dietary protein supplementation significantly increased changes in: strength-one-repetition-maximum, fat-free mass (FFM), muscle size-muscle fiber cross-sectional area, and mid-femur cross-sectional area during periods of prolonged resistance exercise training (RET). The impact of protein supplementation on gains in FFM was reduced with increasing age and was more effective in resistance-trained individuals. Protein supplementation beyond total protein intakes of 1.62 g/kg/day resulted in no further RET-induced gains in FFM. https://pubmed.ncbi.nlm.nih.gov/28698222/
A systematic review and meta‐analysis was performed to determine if increasing daily protein ingestion contributes to gaining lean body mass (LBM), muscle strength, and physical/functional test performance in healthy subjects. The effect on LBM was significant in subjects ≥65 years old ingesting 1.2–1.59 g of protein/kg/day and for younger subjects (<65 years old) ingesting ≥1.6 g of protein/kg/day submitted to resistance exercise, although the increase in LBM in those ingesting ≥1.6 g of protein/kg/day was small. https://pubmed.ncbi.nlm.nih.gov/35187864/
There are some studies that looked at the effects of > 2 g/kg of protein, and even up to 4.4 g/kg. Across these studies, the incremental effect of pushing protein well past ~2 g/kg/day on FFM/lean mass is small to ~zero.