I understand the request and it has popped up previously in many incarnations this is the most recent one:
https://meta.discourse.org/t/trigger-search-onclick-hashtags/75666
I recall a topic from 2013 but can not find it.
The idea is that user a write #banana in a topic and just like Twitter, it magically turns into a link and anyone can click #banana to find all posts with #banana.
The problem you will face here is that # is already taken so you will need another key or fallback logic… maybe ## or something. Other problem is that this twitter like feature would require lots of internal bookkeeping for trending hashtags and discovery and so on.
I don’t see this feature landing in Discourse but someone from the community could experiment with a plugin if they want to. That said I could only imagine very very specific forum where this feature would even be used.