Hypothetical:

The Gestapo says "We waterboarded Valerie Timm until she confessed to being a Communist. We waterboarded her more, until she gave us names of more Communists, and she named YOU. Give us all your correspondence with her, so we can see if you are a Communist too". I say "I corresponded with Val 50 years ago. I tossed all her letters years ago". Gestapo "You destroyed evidence!"

The actual Gestapo weren't subject to U.S. law, and I don't know how that would have worked under 1940's German law.

If you're using "Gestapo" metaphorically to refer to U.S. law enforcement, then none of that hypothetical is possible. U.S. law enforcement isn't allowed to waterboard people, if they did waterboard someone the testimony they get wouldn't be admissible to obtain a warrant, they can't demand your correspondence without a warrant, they can't get a warrant just to see if you're a Communist, and if they tried any of that you could just go to court and get an order telling them they can't search your papers without a warrant. It's a hypothetical, sure - but that hypothetical has no connection to actual U.S. law.

Even then, the statute wouldn't apply. The statute doesn't prohibit "destroying evidence." It prohibits destroying property that is the subject of authorized search or seizure efforts by law enforcement. If you tossed all the letters years ago, you would not be in violation of the statute, even under the scenario applied above.

That's the importance of the presumption of innocence.

How quaint.

Why is that "quaint"? There is a presumption of innocence in U.S. criminal proceedings, and it is generally adhered to. That's the law, the judge will instruct the jury that they are required to presume the defendant is innocent, any legal disputes during the trial will be resolved with that being the operative principle, etc. No doubt there are some jurors who walk into the jury room with some predisposition to assume that if someone was arrested they must have done something wrong, but the formal requirements of the system is that there is a presumption of innocence - and that's likely adhered to more today that it was, say, 60 or 70 years ago.