Why did the FBI disclose that they have footwear impressions from the person of interest? Is that not a "destroy the footwear" banner?

Same thing, but to a lesser extent, about disclosing the obtained palm print and forearm imprint. Though those could also be destroyed, likely not without significant pain.

More likely, to LEO disadvantage, if they find and are surveilling a person of interest, waiting for an inadvertent palm print or "forearm imprint" (however one gets one of those) to be left behind in public, the person of interest will now be more cautious about trying not to leave a palm print or forearm impression, or both, behind in public places.

Law enforcement usually does not disclose evidentiary details, if at risk of destruction, during an ongoing investigation,.