The specification documents ended up in limbo, in a GitHub-hosted export from Google Code when it shut down. It doesn’t get served in a human-readable format. The specification is still used by OStatus/GnuSocial and Mastodon.
http://portablecontacts.net/ went blank in early 2017, leaving the specification documents abandoned. This copy was reconstituted from the Wayback Machine. The protocol is still used by OStatus/GnuSocial and Mastodon in parts, using part of the schema vocabulary to extend the Atom format of the Activity Streams 1.0 spec