This seems to be a planned change related to a new link generator they released in 2017 [1]. You can opt out here [2] if an old video is effected. It seems like

This seems to be a planned change related to a new link generator they released in 2017 [1]. You can opt out here [2] if an old video is effected. It seems like the unlisted feature is otherwise unchanged. I suppose you can also flip an effected video back to unlisted and get a new link after this goes into effect. [1] https://support.google.com/youtube/thread/114633828/changes-to-unlisted-videos-uploaded-before-2017?hl=en https://support.google.com/youtube/thread/114633828/changes-... [2] https://support.google.com/youtube/answer/9230970 https://support.google.com/youtube/answer/9230970