Yeah, worth checking the TOSes:

https://developers.google.com/youtube/terms/developer-polici...

https://developers.google.com/youtube/terms/developer-polici...

Understandable: creators often update their video metadata (title, description etc.) or make public videos back to private or unlisted, and it is in their own right to do this.

Third parties that copy data will cache stale versions even when the creator doesn't want to share them anymore. Seems fair that YouTube doesn't condone this.