Adam Clarke's Commentary makes this comment.
"From Judges 11:39; it appears evident that Jephthah's daughter was not Sacrificed to God, but consecrated to him in a state of perpetual virginity; for the text says, She knew no man, for this was a statute in Israel. בישראל חק ותהי vattehi chok beyishrael ; viz., that persons thus dedicated or consecrated to God, should live in a state of unchangeable celibacy. Thus this celebrated place is, without violence to any part of the text, or to any proper rule of construction, cleared of all difficulty, and caused to speak a language consistent with itself, and with the nature of God."