It was Covenant.
Ridley (or at least his people) had met with Blomkamp's team and were willing to let his film go forward, under the condition that Blomkamp changed one element of the film that didn't mesh with his prequel narrative. Blomkamp's film was to wait until Covenant was wrapped before going into production, but sometime during that process it crumbled and was abandoned. Nobody knows exactly what went down, but based on Ridley's comments it seems there never was a full, locked script for Blomkamp's film in place.