Well, they managed to analyze DNA from centuries-old hair in this paper. From the methods section: > Nuclear DNA in hair has an extremely low average fragment

Well, they managed to analyze DNA from centuries-old hair in this paper. From the methods section: > Nuclear DNA in hair has an extremely low average fragment length owing to the activity of endonucleases expressed during hair formation. But the researchers made adjustments for that during sample prep before sequencing, and alignment after sequencing.