Earlier quoted context omitted.
> Article I of the United States Constitution lists issuing letters of marque and reprisal in Section 8 as one of the enumerated powers of Congress, alongside the power to tax and to declare war. It's still a thing
Reportedly the Russian government has issued letters of marque to hacking and cybercrime groups. I can imagine the same has happened in the US.
Those would only be metaphorical (and even then not a particular apt metaphor) letters of marque and reprisal; a literal letter of marque and reprisal is to a private vessel to attack and seize property from enemy vessels, converting acts which would otherwise be piracy under international law into lawful armed combat.
Cyberhacking groups aren’t vessels, and what they are doing isn’t something that would be piracy under international law without being under the umbrella of government war efforts.
> I can imagine the same has happened in the US.
Not under US law (even if it still wouldn’t make any sense under international law for the reasons described above), since Constitutionally that would require an active of Congress authorizing such letters, which has not occurred. You can imagine what you want, but its simply not a reality.