I had the plugin installed a while ago, so it was an old version. It’s not in any template files. When I updated the plugin and deactivated it, that’s when the error happened.
So I had the old version, active but not being used. Updated all my plugins. Now the new version was active. Still not being used. Then I deactivated it, and got the error. When I activate it, the error goes away.
I think the previous setting was “Automatically display subheadings before post content.” No references to “the_subheading” are being called because I wasn’t using it.