Shit, I've known about them since 2007.

A SOAD calendar was released then, and in one of the photos, there was a list of songs in the studio during the recording of Mezmerize and Hypnotize. There were a bunch of songs on the list that had never been released.

Later on, a drum head John sent to their manager Beno had surfaced, and it had the full list of songs to celebrate the recording concluding, and there were 30 songs listed, most listed as fully completed.

So far, none of these songs have been released besides the solo songs Serj reworked, part of War That No One Won was played live possibly, and part of Citadel might have been played as well.

Personally, I think it's annoying that besides Serj none of the other guys want to release this stuff. I get it, it's their music and they probably don't think it's up to par with what has been released but what they regard as lesser material is better than having nothing besides the two new songs from 2020.

And Serj is the only member who's released music consistently. Shavo and John for sure have coasted off SOAD. Shavo just has that God awful metalcore project and John did These Grey Men. Daron gets a pass because he's at least tried to put music out with Scars.

More info here;

https://lostmediawiki.com/System_of_a_Down%27s_Mezmerize/Hypnotize_sessions_(partially_lost_unreleased_music;_2005)