Ellen G. White specifically addresses this question in her writings. She explains that there is a distinction between slaves who have been kept in ignorance and degradation, and those who have known and served God faithfully.
For slaves who have been kept in ignorance and degradation—knowing nothing of God or the Bible, and living only in fear of their masters—Ellen White writes: "God cannot take to heaven the slave who has been kept in ignorance and degradation, knowing nothing of God or the Bible, fearing nothing but his master's lash, and holding a lower position than the brutes. But He does the best thing for him that a compassionate God can do. He permits him to be as if he had not been..." (Early Writings, p. 276.1). This means that such individuals will not be resurrected to face judgment or suffering; instead, they will simply be as though they had never existed.
However, for "the pious slave"—those who knew and served God—Ellen White describes a different outcome: "I saw the pious slave rise in triumph and victory, and shake off the chains that bound him, while his wicked master was in confusion, and knew not what to do; for the wicked could not understand the words of the voice of God" (Early Writings, p. 286.1; Life Sketches, p. 103.1). This indicates that faithful slaves will be resurrected and delivered at the second coming.
In summary: According to Ellen White, slaves who were kept in total ignorance and degradation will be as though they had never existed at the second coming, while those who were faithful to God will be resurrected and delivered.
References:
- White, E. G. (1945). Early Writings. Washington, D.C.: Review and Herald, pp. 276.1, 286.1.
- White, E. G. (1915). Life Sketches of Ellen G. White. Mountain View, CA: Pacific Press, p. 103.1.