Thanks, @ManishPatel. Creating a web app (or apps) with the Interactive Legend, then adding those apps to the Story Map is a reasonable workaround.
I was thinking specifically of the Story Map "Series" where you can have each tab's location be inherited from the first map; I think the embed method won't do this. Still, what you suggest is a good workaround - thank you.
Hopefully Esri adds a widget for an interactive legend to story maps!