The proposed way: Enable “Apply border” in tab “Options” in Tools > Autocorrect Options. Type in new paragraph followed by enter
- three hyphen
--- to get a 0.05 pt line,
- three underscore
___ or three equal sign
===to get a 1.0 pt line
- three asterisk
*** to get a combined line thin over fat
- three tilde
~~~ to get a combined line fat over thin
- three hash
### to get a double line.
(There might exist more versions, I don’t know.)
The old dialog had two very different kind of horizontal rule: (1) simple border rule and (2) graphic rule.
For (1) you can use the way described above or insert an empty paragraph, open the Style&Formatting window, (F11) or Sidebar. Switch to category “Paragraph styles” and double click on “Horizontal Line”.
(2) Graphic rule had been a Gallery Theme, but that was removed. Resulting problems for older documents, are tracked in issues fdo#67208 and fdo#60858.
If you still want to use these graphics you can try to copy them from an older installation. I have taken those files from an Apache OpenOffice installation and zipped them (=91KB), but cannot attach it here, because of not allowed file format. If you want to use them, drop me a note and I’ll mail them to you. Close LibreOffice and QuickStarter. Goto the LibreOffice installation path, then therein to folder share/gallery. Copy the folder “rulers” and the files “sg2.sdg”, “sg2.sdv” and “sg2.thm” into that folder. After a restart of LibreOffice you should find the graphical rules in the Gallery. Set cursor into the text, where you want the graphical rule, open Gallery theme “Rulers”. Right click on the rule you want and choose item “Insert”. You should break the link to the graphic in Edit > Links, so that the graphic is embedded into the document to avoid, that the graphics are missing, when the file is opened in another LibreOffice installation.