After trying to find the ability to Insert a Horizontal Rule (much like was in OpenOffice.org), I find that it does not exist for some reason.
I have gone to the Help wiki, and followed its instructions carefully, but do not have the "Horizontal Rule" option appearing in the "Insert" submenu.
I use Horizontal Rules a LOT in some of my documents. So far, I have NOT found that option under the insert menu like it should be.
The current, up-to-date version LibreOffice(Writer) I am running is 188.8.131.52, on an Arch-Linux build called Manjaro. It seems all other features are available.
Operating System: Linux (Other)
Version: 184.108.40.206 release
The item has been removed on purpose, see discussion in issue 60858.
For the plain rule you can use the autocorrect options or assign the paragraph style 'Horizontal Line'. For an image-rule you can use any picture and set its width and height manually, for example copy the images from the old Gallery to your new installation.
*** This bug has been marked as a duplicate of bug 60858 ***