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#YouTube is setting unlisted videos uploaded before 2017 to private unless the channel owner opts out.
If you have any old unlisted videos, please opt out ASAP so people can continue to access the videos at the same URLs!
https://blog.youtube/news-and-events/update-youtube-unlisted-links/
YouTube says this is due to a security update that was made for unlisted videos, but they clearly have the technical ability to preserve the URLs of old videos if they have an opt-out option.
"URIs don't change: people change them." https://www.w3.org/Provider/Style/URI