Actually this is one thing of the few things they actually got right. See this article:
https://bible.org/article/my-favorite-passage-thats-not-bible
It is an important point to note that although the story of the woman caught in adultery is found in most of our printed Bibles today, the evidence suggests that the majority of Bibles during the first eight centuries of the Christian faith did not contain the story. Externally, most scholars would say that the evidence for it not being an authentic part of John’s Gospel is rock solid.
How, then, has this passage made it into modern translations? In a word, there has been a longstanding tradition of timidity among translators. One twentieth-century Bible relegated the passage to the footnotes, but when the sales were rather lackluster, it again found its place in John’s Gospel. Even the NET Bible (available at www.bible.org), for which I am the senior New Testament editor, has put the text in its traditional place. But the NET Bible also has a lengthy footnote, explaining the textual complications and doubts about its authenticity. And the font size is smaller than normal so that it will be harder to read from the pulpit! But we nevertheless made the same concession that other translators have about this text by leaving it in situ.
I believe it’s time for us to own up to our tradition of timidity and recognize that this has not helped the Church in the long haul. It’s time to close the gap. I am calling for translators to remove this text from the Gospel of John and relegate it to the footnotes. Although this will be painful and will cause initial confusion, it is far better that laypeople hear the truth about scripture from their friends than from their enemies. They need to know that Christ-honoring, Bible-believing scholars also do not think that this text is authentic, and that such a stance has not shaken their faith one iota. No cardinal truth is lost if these verses go bye-bye; no essential doctrine is disturbed if they are cut from the pages of the Word of God.