IANAL, but I know of a case where it was charged (though dismissed in a plea deal with other charges remaining).
Plus, on federal level, there's this:
https://www.law.cornell.edu/uscode/text/18/4
18 U.S. Code § 4:
"Whoever, having knowledge of the actual commission of a felony cognizable by a court of the United States, conceals and does not as soon as possible make known the same to some judge or other person in civil or military authority under the United States, shall be fined under this title or imprisoned not more than three years, or both."
The reason its an unused law is because it violates the 5th and 6th Amendments the way its being used here. Besides it involves a positive act, concealing a felony is an affirmative action not a passive one, not just having knowledge of it.
It stands just having knowledge of a crime puts no duty on oneself to report it. Going beyond that to actively concealing the crime is a whole different matter.
Criminal Law 501.
Each Crime has elements all elements must be proven to convict
This Code has the Following elements
1. knowledge of the actual commission of a felony
2. conceals, and
3, does not as soon as possible make known
Element 2 Conceals
conceal - verb - camouflage, cloak, confine, cover, cover up, curtain, disguise, eclipse, enshroud, entomb, envelop, harbor, hide, keep clandestine, keep from, keep out of sight, keep secret, keep to oneself, keep underground, make innonspicuous, make indiscernible, make unapparent, make unperceptible, mask, not reveal, obscure, occulere, protect, render invisible, screen, seclude, secrete, shade, shadow, shield, shroud, store, suppress, throw a veil over, veil.
Associated concepts: conceal assets, conceal information, conceal material facts.
The Arm chair lawyers who cited this for the proposition they did are thinking USSR law not American law.