Do you use YouTube's own web-based video editor to cut together your YouTube videos?
Statistically, you probably don't. Google has noticed that less than 0.1 percent of all YouTube creators are using that particular feature, and so it -- and YouTube's Photo Slideshows feature, which I can't say I've ever heard of -- are officially going away on September 20.
You'll have until then to wrap up anything you're working on. But do note that YouTube's "Enhancements" feature, which let you trim, add subtitles, cards and filters, etc, will still work.
YouTube first added the cloud-based video editing feature, in 2010.