That is still clearly intellectual property after 15 years. There could be trade secrets in there. And the slightest suspicion of which is a good reason to sue

That is still clearly intellectual property after 15 years. There could be trade secrets in there. And the slightest suspicion of which is a good reason to sue you, should anyone every need a reason to. It could be an personal conflict spun out of control or an incompetent manager needing someone to blame for a failed business or whatever. If there are any customer data or logs in there, then you could be personally liable under data protection laws. They might not even have an expire date, depending on what kind of data it is. Better safe than sorry, unless you think you have something to gain from keeping that data around.