Isaiah 12:2 (King James Bible) he also is become my salvation
In older English, be was used as the perfective helping verb (rather than the have of modern English) whenever the following past participle (the main verb) involved change or motion. For instance, modern English uses “the Lord has come” while older English has “the Lord is come”. Here in 2 Nephi 22:2, the 1837 edition introduced has as the auxiliary verb for the past participle become, perhaps by accident (the change was not marked by Joseph Smith in 𝓟). There are similar examples in the text of the be verb as the helping verb for the past participle become. Two instances of “art thou become” (found in a nearby Isaiah quote) have never been edited to “hast thou become”:
But in two other examples in the text, an original be has been replaced by have:
In the first instance, have showed up in the 1840 edition. Joseph Smith edited the 1840 edition, so it is possible that he is responsible for the change there (although the have could have been accidentally introduced by the 1840 typesetter). In the second instance, were was replaced by had in the 1837 edition, and for that case the change was marked by Joseph in the printer’s manuscript. This example clearly shows that at least one of these changes was fully intended. Thus it is also possible that in 2 Nephi 22:2, the 1837 change from is to has was the result of Joseph’s editing.
In any event, the original archaic use of be as the helping verb for the past participle become will be maintained in the critical text for all five of these cases since in each case the earliest textual sources support such a reading. Normally, the text prefers have with the past participle become (or the dialectal became), with 59 occurrences, including “they had become exceeding wicked” (in Helaman 6:31) and “they would not have become slippery” (in Helaman 13:33).
Summary: Restore the archaic is in “he also is become my salvation”, the original reading in 2 Nephi 22:2 and also the corresponding King James reading.