hg-ssh is installed with mercurial at /usr/local/share/mercurial/contrib/hg-ssh
It is not executable, and it is not available in the default $PATH. Some software like sourcehut (https://sr.ht) expect hg-ssh to be executable and available.
It also seems to be the case for the case for the other files in /usr/local/share/mercurial/contrib with a "env python" shebang. I suppose that dumprevlog, simplemerge, undumprevlog and hgk should also be executable, and available in $PATH