Miniflux version 2.0.51
Youtube doesn't distinguish shorts from regular videos in their rss feed. The only way to know (without changes to Miniflux) is to enable FETCH_YOUTUBE_WATCH_TIME and filter out videos by duration. This does mean some videos may be caught in the crossfire, but in my case that's not very likely and it's an acceptable loss.
I know Miniflux recently added webhooks but I don't feel like reading into it, the API in general, and setting everything up. Less effort to just create a trigger.