There are two newer meta-analyses that I don't see mentioned above, from 2021 and 2020.
https://pubmed.ncbi.nlm.nih.gov/33034021/ shows there is some gain in muscle mass in older people. See
https://www.nutritionaloutlook.com/v...n-older-adults for a non-technical summary of this paper.
https://pubmed.ncbi.nlm.nih.gov/32456217/ shows there is no strength gain in younger athletes and a small muscle mass gain. See
https://www.mysportscience.com/post/does_hmb_work for a non-technical summary.