He said that during promotion for GotG Vol. 2 ("I don't know if it's Phase 4, it might be a new thing"). Then when he was out there doing interviews for Ragnarok, he didn't specifically state there wouldn't be a Phase 4 - just that everything after Endgame would be very different than what came before. We've been through five MCU movies since all that was first said, so things might have changed.
I've been thinking about it, and the more I do, the less I see them going forward with Avengers team-ups that we're used to. Pretty soon, they'll have all the pieces needed for the Illuminati, and honestly I wouldn't be surprised if they go that route after Endgame and what happened to everyone in Infinity War. The comics reason they used for the need of the Illuminati was how ill prepared they were after the Kree/Skrull war and nobody shared information. That could be applied to the MCU, and how nobody shared information with the others about the Infinity Stones, knowledge of Thanos, etc and the Civil War event splitting everyone up so the odds were against them in the first place.