An interaction on the board in Zavala’s office from Osiris states that it’s mysteriously disappeared. He cannot detect it on any long range scans and Nessus’ orbit has changed (due to the Leviathan no longer being there). During the cutscene that played during the live event, we saw dark tendrils pass over the top of Nessus which could suggest that whatever happened to Io, Titan, Mars and Mercury, happened to the Leviathan as well.

It's hovering around Nessus. It wasn't hit with the Darkness wave as far as I know, but Calus still closed it off to Guardians waiting for the Darkness to arrive.