As discussed under 1 Nephi 22:4, Oliver Cowdery tended to mix up whether and whither in his manuscript work. Here in Mormon 8:10, 𝓟 reads “and whither they be upon the face of the land no man knoweth” while the 1830 edition reads “and whether they be upon the face of the land no man knoweth”. 𝓞 is not extant here, but Oliver (the presumed scribe in 𝓞 for this part of the text) could have written either whither or whether in 𝓞. In any event, according to 𝓟 no one knows where the three Nephites are, while the 1830 edition states that no one knows whether the Nephites are anywhere in the land. The implication of this second reading is that the three Nephites have been withdrawn from the land and are somewhere else on the earth.
The context implies that the three Nephites are still around but that nobody knows where they are. The following text (in verse 11) indicates that they still make appearances to the righteous (namely, Mormon and Moroni): “but behold my father and I have seen them and they have ministered unto us”. The critical text will accept the reading in 𝓟 as being the most reasonable in the larger context. In 1 Nephi 22:4, the text uses whither in a similar way:
Also note from this example that whither occurs with the verb be, which means that in the Book of Mormon whither is not restricted to verbs of motion. Thus here in Mormon 8:10 whither can occur with the be verb.
David Calabro (personal communication) points out that earlier it is explained that “the beloved disciples were taken away out of the land” (Mormon 1:16) and thus the question cannot be where they were “upon the face of the land”. But that passage also argues that no one can ask whether these disciples were “upon the face of the land” since Mormon 1:16 clearly states that they were “taken away out of the land”. Ultimately, it seems more appropriate to interpret Mormon 1:16 as simply saying that the disciples were removed from among the people, so no one knew where they were. In fact, this is what the text states here in Mormon 8:10: “the Lord would not suffer them to remain with the people”. But Mormon and Moroni were visited by them, thus implying that even they knew they were somewhere but without knowing precisely where.
Summary: Restore in Mormon 8:10 the reading of the printer’s manuscript (“and whither they be upon the face of the land no man knoweth”); the context implies that the three disciples of Jesus are still in the land but that no one knows where; the use of whither in 1 Nephi 22:4 also supports this interpretation.