domain-mapping.php - what folder should it go in?

1. What folder does this file go in?
I watched this video, which shows the file was moved to mu-plugins:

However, when I moved the file, all I got was a blank white page for control panel. I had to move the file back in order to get access to the dashboard.

It appears the plugin set up, except there is only a domain mapping link in the settings menu, but no domains link. No place at all to add domains.

2. Also, I had the domain set up with its own WP site, and deleted all the WP files so the domain is still set up in cpanel. Do I need to remove the domain from cpanel, then go to the registrar and park the domain?

3. Will this plugin be discontinued? I don't want to set up my entire network based on a plugin that may not be available in the future. The only other way I could get my domain to work is to forward the domain to the subsite address. I went in to cpanel, did a forward on the domain so it will point to another domain.