It seems the videos were removed automatically. Maybe only now Youtube's software caught that up and the producers forgot to tell that they want to keep the videos.
But that's just a wild guess on how things work there.
But that's just a wild guess on how things work there.
Specifically: What happens when Content ID identifies a match?
If Content ID identifies a match between a user upload and material in the reference library, it applies the usage policy designated by the content owner. The usage policy tells the system what to do with the video.
So, I'd highly suspect that just now the content owner set the policy. I can't believe that the filter is multiple years out of date, and just now catching up. They wouldn't all be flagged at once, if the processing was that slow.