YouTube has been the biggest video-sharing site out there for many years, but that doesn't mean that everything about it is good. Two of those not-so-good things include the Video Editor and photo slideshows, which are both sparingly used. For that reason, YouTube is discontinuing the use of these two features after September 20th, 2017.
A community manager posted in the Google Product Forums' YouTube Help Forum that both of these things would be going the way of the dodo in two months. The reason cited for this decision? Limited usage, and that YouTube wants to focus its efforts on creating new tools and updating other, more popular features. That's understandable, as neither the Video Editor nor the photo slideshows is especially useful. After all, software like iMovie and Movie Maker can do the same things quite easily.
If you've got existing projects in the Video Editor, they'll be erased after September 20th. Published videos will be unaffected. You'll still have access to the Enhancements feature in Video Manager, which enables you to trim, blur, and filter your videos, so not all's been lost. Well, I doubt many of you used these two features in the first place.