In Verse 4:10, Oryx says to Crota [quote]“My son,” he said, “this is your punishment. Come home glorious, or die forgotten!” He picked up Crota by the legs and threw him into the Vex gate network.[/quote] It is possible that this is exactly what happened to Nokris: he was forgotten, wiped from all records, even the World's Grave. Everything except the one statue standing in the Dreadnaught