I've been asked if it is permissible to upload Adobe Stock Videos to YouTube or Vimeo as a way to embed them into a website, rather than uploading them to the website's server.
I had a look through the terms and conditions and I don't see anything prohibiting this, but I'd very much appreciate confirmation that this is an acceptable practice.
No, this would not be acceptable. The terms state you must take all reasonable precautions to prevent the images/videos from being directly downloaded on the web. Adding them to Youtube as is without any edits to them would allow the videos to be downloaded by anyone in the .mp4 format.
It wouldn't make sense to do it that way anyways as they would have the Youtube player controls around them as well as they would need to be started manually.
It's actually easier to copy a video that's running from a website server than it is to capture the stream from an embedded YouTube or Vimeo video, but neither option offer any real protection. It also makes sense to use YouTube or Vimeo on shared hosting, for example, as they then do the heavy lifting of streaming the content.