https://www.vice.com/en_us/article/88xxex/youtube-tells-open-source-privacy-software-invidious-to-shut-down

YouTube has sent a cease-and-desist letter to Invidious, an open-source “alternative front-end” to the website which allows users to watch videos without having their data tracked, claiming it violates YouTube’s API policy and demanding that it be shut down within seven days. 

“We recently became aware of your product or service, Invidious,” reads the letter, which was posted on the Invidious GitHub last week. “Your Client appears to be in violation of the YouTube API Services Terms of Service and Developer Policies.” The letter then delineates the policies which Invidious is accused of having violated, such as not displaying a link to YouTube’s Terms of Service or “clearly” explaining what it does with user information.