User blog comment:Just a Guy That Likes Creepypastas/Subscriptions?/@comment-25428589-20190220120438

If the author has an author category, you can follow their category page, and you'll get emailed whenever a new story is added to that category. There's a list of user categories here.

I guess the process would be as follows: Find the author category (e.g: Category:EmpyrealInvective) then go to Special:EditWatchlist/raw and add the page title as a newline at the end.