The subordinate conjunction that was removed here in the editing for the 1837 edition, probably because it leads the reader to expect a main clause after the following after-clause. Elsewhere, whenever the text has even that, the following clause is a main clause and stands on its own. The text has 17 examples, including this nearby one:
In other words, “all my joints are weak” is not a subordinate clause like “after many generations have gone by them”. On the other hand, there are three examples of even followed by a prepositional phrase headed by after, such as “and even after all this / they shall consider him as a man” in Mosiah 3:9 (for the two other examples, see Helaman 8:18 and Ether 3:17). But there are no other examples of “even (that) after ”—that is, both the original and the edited constructions in 2 Nephi 3:20 are unique. Despite the uniqueness of the earliest reading, it seems that the that is intended, however awkward it may sound to modern-day readers.
Summary: Restore the that which appears in the earliest text for 2 Nephi 3:20 (“even that after many generations have gone by them”).