A literal interpretation of the claim is, quoted directly, "old code has fewer bugs than new code". Now you've clarified that you meant "stable code", which is fair. But stability is not a guarantee of correctness. Perhaps you meant "correct code", and not necessarily "old" or "stable" code.
I admit it is difficult to infer your intention on the classification of "old code vs new code". I don't see evidence that new code is inherently worse than old code, including in the cited study. There is correlation that people forget lessons learned in the past, but that's as far as I would evaluate the data.
> vulnerabilities decay exponentially.
Because they get fixed over time. That isn't controversial. They also persist over time. The quoted observation depends on proactive maintenance. But proactive maintenance is also the only cause of new bugs by definition. "Recently modified code" stands out in the last paragraph. This study is not talking about stable code.
I admit it is difficult to infer your intention on the classification of "old code vs new code". I don't see evidence that new code is inherently worse than old code, including in the cited study. There is correlation that people forget lessons learned in the past, but that's as far as I would evaluate the data.
> vulnerabilities decay exponentially.
Because they get fixed over time. That isn't controversial. They also persist over time. The quoted observation depends on proactive maintenance. But proactive maintenance is also the only cause of new bugs by definition. "Recently modified code" stands out in the last paragraph. This study is not talking about stable code.