The smh.sh domain referenced in the url and docs links in the README is down, as is the get.smf.sh domain referenced in the installation instructions.

Hypothetically, you could change the installation instructions to:

curl -L https://raw.githubusercontent.com/sm/sm/master/bin/sm-installer | sh

This doesn't look as good, but it could work, if that file worked. In practice, it doesn't because that file internally also refers to https://smf.sh/zsh/$platform/zsh to retrieve zsh, which also won't work. It doesn't check whether you already have a compatible zsh installed on your system first, before trying to download another one.

It might still be possible to install from a local checkout of the repo, though I haven't tested this. There seem to be a lot of internal references to smf.sh domains, so I don't expect sm to work even if it installs correctly.

I don't know what else was on the missing smh.sh site though, or where to retrieve that information now.