These sections and configurations are related to legacy security. Legacy security was deprecated some time ago and removed on WildFly 25. In WildFly 26 you have to use Elytron to configure how a Secondary Host Controller (slave) can connect to a Domain Controller (master). Notice we are starting using new terms instead of master/slave because they are no longer appropriate and will be obsoleted on WF27.
You mentioned you want to add a new WF26 to an existing cluster on a domain mode configuration as a remote secondary host. Take into account that your Domain Controller must be running also on WF26 if you want to manage a WF26 as a remote host. In general, you should always have paired versions, domains with different versions are not recommendable on WildFly.
Having said that and assuming you have already upgraded the hosts on your domain controller and all of them are running WF26, you can use the following steps to configure a user to register a new WF26 remote host on your domain:
1. On the domain controller, creates a user as usual by using user-add.sh script.
2. Configure your secondary host controller to connect by using that user.
The following configuration assumes you have added a user with the name "username" and with the password "userpassword", and the hostname of your remote host is "your-remote-hostname". You have to modify them accordingly to your environment:
embed-host-controller --std-out=echo --host-config=host-slave.xml
/host=your-remote-hostname/subsystem=elytron/authentication-configuration=secondary-hc-auth:add(authentication-name=username, credential-reference={clear-text=userpassword})
/host=your-remote-hostname/subsystem=elytron/authentication-context=secondary-hc-auth-context:add(match-rules=[{authentication-configuration=secondary-hc-auth}])
/host=your-remote-hostname:write-attribute(name=domain-controller.remote.authentication-context, value=secondary-hc-auth-context)
stop-embedded-host-controller
Save the above in a file, e.g configure-remote-host.cli.
3. Execute the file on your remote host controller:
$WFLY_HOME/bin/jboss-cli.sh --file=configure-remote-host.cli
In any case, your first step is to upgrade your domain if you are lagging behind, and always try to keep your server running on the latest versions.