The original Book of Mormon text has the singular verb is and the plural covenants. There have been two possible solutions to this grammatical difficulty: (1) change the verb form to the plural are (beginning with the 1840 edition and continuing in the RLDS textual tradition, and also adopted by the 1920 LDS edition); (2) change the noun to the singular covenant (beginning with the 1852 LDS edition and continuing in the LDS text up to the 1920 edition).
The singular it in “it meaneth” could be interpreted as referring to a singular covenant. This interpretation would imply that the plural covenants was another case of the scribe accidentally (and unnecessarily) adding the plural s to a word. More likely, however, the singular pronoun it is a more general reference to Isaiah’s prophecy of how the Gentiles would assist in restoring the house of Israel. The following verse also begins with “it meaneth”, and in that case it seems to refer more generally to the prophecy itself (“and it meaneth that the time cometh that …”).
A stronger case can be made for the plural “covenants of the Lord”. There are 40 occurrences of the singular “covenant of the Lord” in the King James Bible, but none in the Book of Mormon. On the other hand, the Book of Mormon has only the plural “covenants of the Lord” (11 occurrences, counting this one in 1 Nephi 22:6), but none of “covenant of the Lord”. Thus usage elsewhere argues that the original phrase “for thus is the covenants of the Lord with our fathers” meant ‘for thus are the covenants of the Lord with our fathers’. The use of is followed by a plural noun is quite common in the original Book of Mormon text. This kind of usage is found, for instance, in the original text for 2 Nephi 10:21 (“great is the promises of the Lord”) and Alma 11:22 (“here is six onties of silver”). For additional discussion, see subject-verb agreement in volume 3.
The standard text can therefore continue to follow the plural verb are in “for thus are the covenants”, but the critical text will restore the original singular verb is.
Summary: Maintain in 1 Nephi 22:6 the plural covenants and restore the singular verb is in “for thus is the covenants of the Lord with our fathers”.