Document retention policies are complicated in how they deal with legal issues, but ignoring about a million subtleties and complications, you can delete whatever you want if there is no reason to believe it is associated with a crime, and are not doing it to possibly hide evidence.
In the days when everything was on paper, keeping everything around was expensive. Do you really need that memo from 10 years ago? How would you even archive and store it? Deleting it makes sense, but you need to be sure you aren't hiding criminal activity. Or at least don't have the intent to hide criminal activity.
So companies have rules like "delete everything after a year"--there is no intent to cover up a crime. But as soon as you become involved in legal proceeding, you have to start saving stuff.
It gets complicated fast, but that is the basic rule.
Not a lawyer, but I'd expect it to be illegal if it happens while they're under investigation or trial. Could be tempering with evidence? But in "normal" days, it would be strange to be illegal to delete anything.
That sounds like it should be illegal. Would it be illegal? (I'm not a lawyer).