In year 1 of Destiny 2, I completed all vanilla adventures from year 1, but I deleted the rat king quest in year 1 vanilla, before the nightfall changes. Now in year 2 I can't play the adventure to get the quest, because Sloane does not have the vanilla adventures in her menu. So, that makes Rat King unobtainable in year 2. Sloane needs her vanilla adventures put in her menu to progress. Thanks.
Hello, The mission is Enemy of my Enemy mission on Titan. If you didn't do it in year one it's still available on Titan.
All you need to do to get it back is play activities with a fireteam member while they are using the RK. I have personally seen it work in raids and NF.