To be clear: the topic excerpts feature is not horizon-only. Any theme can enable it via the theme-modifier. Many themes do this.
The site setting was the old way to do this, before theme modifiers existed. It was unhidden today because we briefly thought that it was the only way to make it a toggle-able option for users of horizon. But in fact, there is a way to make theme modifiers link to theme settings, which is what we’ve now done in horizon.
We’ve also documented that strategy, so that it’s clear how other themes can do it. (And so we don’t forget about it ourselves 
)