Tech has become reluctant to speak about features on the album because he mentioned one in an interview, and then "lost a big rock feature" because of it. It's speculated that it's the Doors track, either in whole or in part.
That being because he has since confirmed it's not Serj, and the interview that he re-confirmed the Doors track (since it was already reported last year) was pulled quickly and then put back up without the comments regarding the song. Plus, the Doors and Serj were the only two "confirmed" features. He had already previously stated that Corey was unable to do it because of timing and only mentioned James Hetfield as a possibility that he might reach out to.
Some are speculating that he just lost the Jim Morrison vocals for the chorus. I guess we'll find out what happened at some point.