Multisite for Elgg provided a single control panel for installing Elgg on your subdomains.

I think this plugin is no longer actual for Elgg.

Now you can easily do this with a set of tools from Docker, Vagrant, Ansible, etc.

Also, Elgg has no multisite feature since v3.

Look at our platform.

There we use (yet) the ability to run Elgg distributions on subdomains.

This is controlled by our custom plugin and scripts on the server.

So you can create something similar for yourself.