This idea is taken from @NicoleBraun in her post New story maps text/format functionality. I'm breaking this out specifically because the functionality is important and super useful across many types of StoryMaps. Her explanation is below.
Superscripts and subscripts are not available. Seeing as though we use our story maps to jazz up scientific reports, this often creates bulky footnotes for us that take away from our stories, specifically if we can't change text size either.
Hopefully this can be implemented in the next update or two.