Come Follow Me (2020) print edition interprets the "curse" to be "dark skin."
Book of Mormon 2020: Come, Follow Me—For Individuals and Families (Salt Lake City: The Church of Jesus Christ of Latter-day Saints, 2019), 24
2 NEPHI 5:20-21
What is the "curse" that came upon the Lamanites?
“The dark skin was placed upon the Lamanites so that they could be distinguished from the Nephites and to keep the two peoples from mixing [see 2 Nephi 5:21-23; Alma 3:6-10]. The dark skin was the sign of the curse. The curse was the withdrawal of the Spirit of the Lord [see 2 Nephi 5:20]. . . . Dark skin . . . is no longer to be considered a sign of the curse” (Joseph Fielding Smith, Answers to Gospel Questions, comp. Joseph Fielding Smith Jr. [1960], 3:122-23).
Note that the Church issued an apology for this "error" in 2020 print edition and asked Church members to "disregard this paragraph." This paragraph did not appear in the online version of the manual.