Select it in the sidebar, then hit the Delete key. Or choose Edit > Delete from the menubar.
Note: you can delete multiple feeds, and you can delete folders. You can also undo if you change your mind.
Since NetNewsWire is a nights-and-weekends project, we don’t have enough time to run and test on older versions of macOS. Most of the time it will require the most recent macOS.
This was a difficult decision. We didn’t want to ship with no syncing at all, but we also didn’t want to delay shipping until we’ve done a whole bunch of systems.
So we chose Feedbin, since that’s what we use, and since the folks at Feedbin have been friendly and helpful.