The reading separated (spelled as seperated ) is found on a small extant fragment that originally came from the bottom of page 29 of 𝓞. This fragment can be seen still barely attached at the bottom of a gathering of sheets in the ultraviolet photographs at the LDS Church Historical Department. A legible ultraviolet photograph of this now-separated fragment (along with other fragments from the original manuscript) can be found in folder 80, box 2.
When he copied from 𝓞 into 𝓟, Oliver Cowdery misread the word seperated as rejected. Not only do these two words have the same visual contour, but separated is the next-to-last word on page 29 of 𝓞. As he flipped the leaf of 𝓞 over to page 30, Oliver may have read the last part of the line too quickly.
Elsewhere in the Book of Mormon, people are separated as a result of sin and judgment. There are no other textual examples where the verb reject is used to describe the rejection of one group of people by another group. And although the wicked can be “rejected from” the righteous (even though the pronoun from instead of by seems very strange here), it is very difficult to conceive of how the wicked can be “rejected from … that tree of life”. Such an expression just doesn’t make sense: the tree is not an animate being that rejects unacceptable recipients. In fact, an earlier verse expresses the same idea that the wicked are separated, not rejected, from the tree of life:
Thus the original reading in 1 Nephi 15:36 is consistent with usage elsewhere in the text.
Summary: Restore the reading of the original manuscript in 1 Nephi 15:36 (“the wicked are separated from the righteous and also from that tree of life”); the parallel passage in 1 Nephi 15:28 supports this reading.