I am not suggesting that Christians follow the practice of the Jews, I just understand why it is done. As far as Him being unknown God, in the synagogue during Torah reading, HaShem is just substituted for the name of God during the reading of the Torah, but to the reader it is clearly spelled out. In other places Adonai may also be used which means Lord. In fact, the reader of the Septuagint is likely to see the name of God more than the reader of the English KJV Old Testament as the name of God is frequently covered over with the word "LORD." You will see two spellings of Lord in the Old Testament. When you see "LORD", check your Hebrew Bible and you will see the name of God.
This same practice of substituting for the name of a Divine person even happened in the New Testament. "The Word" as the beginning of the Gospel of John is said by one writer to be the substitute name for Michael the Archangel who was considered by some ancient sages as Divine.