In Judaism, the account of Jephthah is considered a combination of legend and mythology. Judges is part of the Deuteronomistic History, compiled centuries after the supposed events (likely during the 7th-6th centuries BCE). The stories may reflect historical kernels but are highly shaped by theological and political agendas of the day.
One of the main Jewish theological theories is that the tale of Jepthah's daughter, known as Seila in Jewish tradition, is a feminine counterpart or moral reflection to the Abrahamic narrative of the Binding of Isaac. This is preserved in the composition known as Biblical Antiquities, composed by the anonymous author often referred to as Pseudo-Philo, writing somewhere between 25–135 CE or just after the destruction of the Second Temple in 70 CE. The author reimagines and expands upon narratives from the Hebrew Scriptures from Genesis through 1 Samuel, and discusses the story of Seila in chapter 39-40.
Whereas Genesis displays Abraham's inaction to God's command to offer Issac as a sacrifice in the shadow of a debate for the lives of the people of Sodom, most of whom Abraham had never met, the narrative of Seila in BA seems to try to redeem God's inaction in the Book of Judges when Jephthah offers up his daughter.
The point is that Seila is considered offered as a literal sacrifice in much of Jewish thought, so much so that the Masoretic Text seems to describe Jephthah's death in Judges in a providential manner, one that the New World Translation rendering committee (or at least the current revision group) seems to be uncomfortable with.
While there is still some considerable debate in Judaism as to exactly what the narrative in Judges is exactly describing--whether this is a literal sacrifice or not (it is still considered a legend, no matter what)--what happened to Jepthah for his rash vow is quite clear. According to the Hebrew Bible (and the way it is understood by Jewish readers and scholars), Jepthath became quite ill, died, and lost many of his limbs as a result of this divine punishment. This led to his various limbs being buried in various locations, which is why the text at Judges 12:7 reads that Jephthah was
buried in the cities of Gilead.--Italics added.
The 1984 edition of the NWT does indeed note this in the footnotes as the reading of the Masoretic Text, but the newly updated "silver sword" has no references whatsoever, showing no alternative readings here. This is particularly odd since this is the preferred Jewish reading and the footnote reading of the NRSVue. (The text can at least be read "he is buried in [one of] the towns of Gilead," but the word for "town" or "city" is indeed plural, and the tradition behind his manner of death is one of the interesting Jewish tropes behind the folklore.) Removing all notices to the plural here is curious.
It has also been the subject of art for centuries.
Behold--"The Sacrifice of Jephthah's Daughter" by Thomas Blanchet, from the Museum of Fine Arts of Lyon, dates from the 17th century. Depicted is an actual sacrifice similar to that of the Binding of Isaac.
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Legends and Myths
Are legends and myths true in Jewish thought? True, yes. Fact, no. For instance, the Torah narrative that God created the world in 6 days is considered a "myth." It is also considered true, in Judaism, that God created the world. But myths use metaphor, not history, to teach. So it is not fact, that is in the Jewish theological or historical sense, that God created the universe in 6 literal days. There are differences between myths, legends, folklore, and history too. The Bible has very little history. In fact, only around the events of the Babylonian Exile does it touch upon history, but it rarely records it. The Hebrew Bible is generally concerned with religion and mostly liturgy--which is probably the most important of all.
The Christian Bible is concerned mostly with catechesis. It does contain some history, but mainly in the epistles and then coincidental, mostly when the author does not intend to do so. In the New Testament there is no "true" or "fact" in the Hebrew Bible sense but a concern for what Christian writers believe is "truth" or salvific enlightenment.