I'm trying to find if there exist a list of words who cannot be written (forbidden - or highly avoided) in US legal documents ?
No. At least not from a legal perspective.
"Legal documents" is an extremely broad term, but the most common "legal documents" are private contracts.
Under U.S. law, there are plenty of clauses that won't be enforced by courts--but they are determined by their meaning, not their language. There are no specific words that will render a contract unenforceable. If you write a document that says, "I hereby agree to sell that asshole Lou my fucking car for three thousand goddamn dollars," and you both agree to those terms, you have an enforceable contract, and a court will, if necessary, treat it like any other contract. On the other hand, if you write "I hereby covenant not to resell my house to any persons of African-American descent," nobody is going to take issue with the vocabulary you're using--but your meaning renders the document unenforceable.
Of course, if you are dealing with legal documents in a professional capacity, you will want to write your contract in professional English, just like you would in a professional letter, or any other kind of document. But learning to write professionally is not specific to "legal documents" and is outside the scope of this site.