US constitutional law prohibits the introduction of evidence obtained illegally.
https://en.wikipedia.org/wiki/Exclusionary_rule ("Exclusionary rule")
There's no "retroactive" exception. The core point of this rule is to deter police from intentionally violating people's rights, under the expectation that what they find will, "retroactively", vindicate them. Won't work.
How would you know when it did? You can’t “retroactively” justify an arbitrary search under the exclusionary rule, but this doesn’t exclude evidence tangential to a legally-executed warrant during the execution of that warrant. For example, suppose someone is suspected of illegally possessing wildlife. A search warrant is issued on the residence. No wildlife is found, and in fact no wildlife was ever on the premises. If officers find large quantities of cocaine during the search, they aren’t precluded from making an arrest, because the warrant used to gain entry and conduct the search was legal.
https://en.wikipedia.org/wiki/Plain_view_doctrine
> In Arizona v. Hicks, police officers were in an apartment investigating a shooting and suspected that a record player in the apartment was stolen. The officers could not see the serial number, which was on the bottom of the record player, so they lifted the player and confirmed that its serial number matched that of one that had been reported stolen. However, the Supreme Court ruled that lifting the record player constituted an additional search (although a relatively nonintrusive one) because the serial number was not in plain view.
There were laws in Germany to prevent what Hitler did. It still happened.