According to a recent post in the Google product forum made by YouTube Community Manager Marissa, the company will be shutting down its Video Editor and Photo Slideshows on September 20th, 2017 due to low usage.
If you have been using the editor you will be able to edit and finalize projects until said date, after that, the feature will be shut down and users will no longer have access to their work in progress projects, however, already published videos that have been edited with the editor will not be affected.
What will remain is the Enhancements feature in the Video Manager which allows you to do minor adjustments like adding filters or trimming clips.
Despite the low interest in the feature, people are not happy about the news. Users on the forum pointed out that the online Video Editor was the only way to edit video on a Chromebook.