But do they keep it somewhere for further analysis later (very useful medically if you want to backtest things) or do they programmatically destroy it?
From a DNA perspective one can only use the Guthrie test samples for about a decade after collection so yes, in a worst case abuse scenario we have cards that could be mined for DNA across a bunch of <10 year olds.
Why is that? Is it because of sample degradation of some kind?