There might be a proof that you bought them, but there is no way to prove that you still own them, there's always the proverbial boat accident.
I doubt the boating accident defense would hold up in court. If they prove you bought bitcoin, you'd need to convince them you lost it, the burden would be on you at that point.
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Or you can tell them that you spent it on a trip to El Salvador. What? having some fun is illegal now?
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