I can confirm that this works in El Capitan, but it is a much harder hack now. To add the ABRelatedNamesConfig.plist file to /Applications/Contacts.app, you need to disable System Integrity Protection (SIP).
1. Boot into Recovery Mode (hold ⌘-R during boot).
2. Start the Terminal app from the "Utilities" menu.
3. Disable SIP and reboot by entering the following command:
csrutil disable; reboot
4. Having disabled SIP, create and copy
ABRelatedNamesConfig.plist
as before
5. Reenable SIP:
csrutil enable; reboot
The example file way back near the start of the thread has one error: the ISO country code for the United Kingdom is GB (Great Britain). Using UK does not get the related names effect.
-- T.M. Wong