My network runs the latest version of WordPress. I noticed that after creating a new subsite, you can go to Network Dashboard > All Sites > click 'Edit' link under the relevant subsite, and change the Domain setting from subdomain.example1.com to example2.com. It works!
Tested on sub-domain multisite setup. So, the question is, am I missing something here? or is the plugin no longer necessary for subdomain multisite setups?