We have a vertical rule who show number. Sometimes, a horizontal rules with column number can be usefull.
Can you add a horizontal rules in QCodeEdit ?
the issue with this request is that first a horizontal rule is not as general purpose as the other builtin panels so it does not necessarily deserve to be builtin.
hopefully, creating one would be easy, provided you have a precise idea of what it should like like and how it should behave (if it interacts) : just create a new QPanel subclass and reimplement paintEvent() and any other event handler you may need.
Having a look at the existing panels (in lib/widgets/) should be enough to get you started. If you need any hints don't hesitate to ask questions (the mailing list would probably be the most appropriate place for this).
Another issue that comes to me is that QCE does not impose fixed width fonts (though it prefers them) and such a panel would only make sense on editors using fixed width fonts which is another arguments in favor of an "external" implementation (but it could then be added as an example/demo so it would be equally re-usable).