That is
original WCF, and not the text of most American denominations. There is curious history (and someplace a WTS master's thesis) as to how the phrase was dropped. But somewhere along the line, a man married his brother's widow, and was charged with sin. The short story is: the phrase was dropped.
A longer historical context is this: Henry VIII's first wife was previously married to his brother, Arthur; he died, and Catherine, eventually a close relative of the Holy Roman Emperor, Henry took to wife. He received a papal dispensation to do so, since canon law (based on interpreting Lev.18:16) forbade it. (per
https://en.wikipedia.org/wiki/Catherine_of_Aragon)
When Henry wished to divorce Catherine, part of his argument was that his marriage to Catherine was a sin, and unlawful. One might argue that in English terms, it was an important point (politically and religiously) to maintain that the Bible forbade such marriages; and the subsequent divorce and all that followed
including the break with Rome and subsequent church reformation efforts was the good fruit of royal and national repentance.
One is free, in WCF-American terms, to believe that the missing phrase is true and biblical, and be convicted by it (original WCF prooftext, Lev.20:19-21
https://reformed.org/documents/wcf_with_proofs/index.html). But one is also free from being bound to confess it is biblical--no exception required. But, if one's subscription is to the unaltered WCF, and one does not believe this is a proper interpretation of the Bible, he should take exception.