When features get added to PHP there’s a lot of discussion first about the new functionality. This is done on the so-called internals mailing list. You can try to follow the discussions via a rather ugly interface at http://news.php.net. The site looks like a very old school web email client where all conversations are just running through each other.
The externals.io site provides a clean interface to the discussions happening on the php.internals list, including nested threads and the ability to track unread messages. It definitely makes it simpler to keep track of the line of emails in a thread, but it's definitely read-only with no intent on being a full on mailing list replacement.