Using this method for private or deleted content exists in a legal gray area, but for unlisted videos that were intentionally shared publicly at some point, it is generally considered acceptable.
: Many creators use unlisted settings for internal reviews or sensitive sharing, mistakenly believing they are as secure as private videos. Conclusion see unlisted videos youtube extension
agervold/youtube-relist: A Chrome extension for displaying ... - GitHub youtube-relist. Lists youtube videos that are unlisted. Using this method for private or deleted content
Once granted, they can: