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The "Help Center" link from the email: https://support.google.com/youtube/answer/9230970
So it seems like they made a weak algorithm for generating links for unlisted videos meaning you could've brute forced the links to find those which were not shared with you and then they decided to strengthen the algorithm but they are not taking into account that many of those videos were "set and forget" as unlisted so they will be automatically switched to private without uploader knowing it leaving shared unlisted video links dead.

It's good that they updated the rng algorithm for generating unlisted video links but the way of handling old unlisted video links is not good.