The SshKeyAuthenticator core plugin is not included in the 3.2.4 release:

$ wget --quiet https://github.com/Internet2/comanage-registry/archive/3.2.4.tar.gz
{{ $ tar ztf 3.2.4.tar.gz | grep -i sshkeyauthenticator}}

It may not have been included in any release going all the way back to 3.0.0.

It is in develop:

$ wget --quiet https://github.com/Internet2/comanage-registry/archive/develop.tar.gz
{{ $ tar ztf develop.tar.gz | grep -i sshkeyauthenticator}}
{{ comanage-registry-develop/app/Plugin/SshKeyAuthenticator/}}
{{ comanage-registry-develop/app/Plugin/SshKeyAuthenticator/Config/}}
{{ comanage-registry-develop/app/Plugin/SshKeyAuthenticator/Config/Schema/}}
{{ comanage-registry-develop/app/Plugin/SshKeyAuthenticator/Config/Schema/schema.xml}}
{{ comanage-registry-develop/app/Plugin/SshKeyAuthenticator/Config/routes.php}}
{{ comanage-registry-develop/app/Plugin/SshKeyAuthenticator/Console/}}
{{ <snip>}}