The 1892 RLDS edition accidentally dropped the word death in the relative clause at the end of verse 11. The result is not at all felicitous. The 1908 RLDS edition restored the original reading (“which death is the grave”). The original reading parallels the relative clause at the end of the next sentence (“which spiritual death is hell”).
The close parallelism between these two sentences suggests the possibility that there might be errors in this passage. For instance, the first sentence reads “which is the temporal”, which may be an error for “which is the temporal death” in light of the parallel relative clause in the second sentence that reads “which is the spiritual death”. Or perhaps the relative clause at the end of verse 11 should read “which temporal death is the grave”, given that the last relative clause in the second sentence reads “which spiritual death is hell”. We could thus complete the parallelism by emending the text to read as follows:
Nonetheless, the earliest text, despite its cases of ellipsis, is acceptable, while the missing death in the 1892 RLDS edition is not. Thus the critical text will here follow the earliest reading. Book of Mormon passages have varying degrees of parallelism. But without specific evidence of an error, there is no need to emend a passage simply because we want to increase its parallelism.
Summary: For 2 Nephi 9:11–12, we accept the earliest attested reading rather than expand the text to increase the parallelism; the omission of the word death in the 1892 RLDS edition is clearly a typo.