Apocalypse should be stranded in the temporal purgatory because he can't rejuvenate anymore because according to TOS the X-Men destroyed his rejuvenation chambers thus he's unable to exist outside of said realm.
Spoiler!
Maybe the X-Men that have traveled back and met Apocalypse in his pre-Apocalypse form created a branch Apocalypse we saw on Genosha that the TVA can't prune due to it being in the midst of upheaval currently at the very end?