I'm probably one of the few who uses this, but I use it a lot. After my upgrade to LibreOffice 4.0, the Horizontal rule menu item is missing from the Writer Insert menu. Where did it go? How can I get it back?
It seems like the horizonal rule may have just been a border style applied to a paragraph, and now that is more official as it appears as a paragraph style.
So you can achieve the same thing by clicking the pararaph you want to be horizonal rule, and clicking "Horizonal Line":
Once you have done this once, you can then select it from the styles quick dropdown. Once you get used to this way of thinking, the horizonal line is just another style, and you'd set up whatever types of line you are after.
You could enable the "borders" icon from the Frame toolbar to get quicker access, or use the old trick of entering three hyphens, tildas or underscores, which auto-correct will convert to a horizonal line with different thicknesses.