No I agree, it should be discoverable better. I’m not sure where it belongs though: it’s not a paragraph format because you can apply paragraph format to the horizontal rule
It’s not a character format (along with bold, underline etc) either.
It could be a separate button on the panel, along with the link. But I guess Coda wanted to limit the buttons to only the most important ones, so the line was an outlier.
And the plus button goes away soon, in favor of slash commands and Explore.