The developers never really did say that a sequel to Shadow of Mordor was going to be made but most people believe that there is.
Rumors started way back when the first game was released. The developers talked about how interesting the new Nemesis system was and they also said that they will be adding a lot more features in the future. Many fans believe that they meant the sequel when they said that.
Fans got even more excited when a stunt actress accidentally listed the Shadow of Mordor 2 in her resume. The game was removed later on but the damage has already been done. Or so we thought.
That “leak” at that time indicate that a sequel is happening. We were expecting to see the game getting announce at E3 this year but that did not happen. The game was also missing from Gamescom 2016 as well. At this point, nobody really knows if a sequel is going to happen or not but we sure hope it is.