In this example from 2 Nephi 9:16, the subordinate conjunction that follows the parenthetical expression, which in the above citation is set off by dashes (the editions have used commas). One could argue that by using dashes in the printed text, this passage could read without the that:
But in this complex construction, the original that-clause actually stands for the pronoun it that is used twice in the preceding parenthetical statement (“for the Lord God hath spoken it and it is his eternal word which cannot pass away”). In other words, the reader expects the that because of the preceding “hath spoken it”. It is true that by setting off the parenthetical statement with dashes, one could then remove the that, thus preventing a sentence fragment. But the resulting construction would be more difficult to process. The critical text will, of course, retain the original use of the that, just as it has read throughout the history of the text.
Summary: Maintain the occurrence of the that after the parenthetical statement in 2 Nephi 9:16; this subordinate conjunction helps the reader interpret the that-clause as referring to the preceding pronoun it, found twice within that parenthetical statement.