This kind of seems to have fallen off the table. There was a lot of hype and it seemed like it was almost ready to be released. The last update I could find was on a Monday update on 5/31 by Ryolnir:
Based on community feedback, we're delaying the Cosmic Ultron game mode and instead choosing to focus on a new Terminal of the new game experience, Times Square, where the heroes battle Ultron. This won't be coming this week for the anniversary, but we're hoping to have it out relatively soon (perhaps with the release of the Ultron playable hero later in June).
So is this still in the 'pipeline'? Or has this gone for now? I actually don't know anyone that actually runs Ultron Mode anymore. Never see anyone in guild chat asking about it, and no one looking for people in LFG channel for Ultron Mode. (Maybe some run it during Mystic Mayhem for the Ultron kill?)