We decided to discontinue the Series Editor beta. The Series Editor beta was only available to a few creators and wasn’t often used. We’re removing it to focus on other features for creators.
You can still create playlists and add series metadata to videos to help viewers watch your videos in order. For example, you can add “S2E3” (season 2, episode 3) to the end of your video title, or to your custom thumbnail. Learn more about playlists on YouTube.