Site Specific MU Plugins?

Hello,

This article on WPMU.org references a "Site Specific MU Plugin" which allows you to install a plugin for a specific site (instead of it being available for all websites):
https://premium.wpmudev.org/blog/site-specific-mu-plugins/

The article says you can download the plugin from http://wpmudev.org/, but I can't find that plugin?

How can I install a plugin for a site but not make it available for all sites?

Thanks