I have a channel on Telegram, a messaging app. It's like a blog, where I post messages to my channel subscribers. Which is the best practice to tag a post: #short_story, for example, or #short_stories?
This is a bit on the primarily opinion based side, but think about this: a hashtag is a metadata tag that you use to provide a taxonomy to contents. For example, think of a library bookshelf: you will have hashtags like Architecture, Drama, Novel, Poetry, and other tags in plural, like Kids, Best Sellers, Classics, and so on.
You’ll notice that IN GENERAL, when the subject has more significance , it goes in singular, and when the subject is more diffuse or it can be included in a broader taxonomy, then it goes in plural. Thus, you may have
Drama —> Best Sellers.
So, in your case: if your posts include a single story, then it will probably be better to use the short_story hashtag. However, if your posts have mixed content, then use the plural