Microsoft Word allows me, helpfully, to assign, say, the paragraph mark to a keyboard shortcut. Writer - and the rest of LibreOffice, I think - do not. Apparently an extension provides the functionality. (See https://ask.libreoffice.org/en/question/7663/lo-writer-insert-special-characters-by-shortcut-keys/) It seems to me a poor show that one should have to rely upon an extension. Also - though perhaps a badly performing search mechanism is to blame - the extension appears no longer to exist.
No, this is already done with macro assignments:
1. via an AutoText <Ctrl>+F3 "macro" assignment, and "macro"+F3 replacement
2. or via Tools -> AutoCorrect -> AutoCorrect Options: Replace table entries (our method for emoji entry).
3. or via <left-alt>+X toggle of unicode glyph, e.g. U+00b6 -> <Alt>+X => ¶
Also a bit GUI'ish, but the LibreOffice Special Character dialog is fully keyboard navigable and now includes reuse of Recent Characters (<alt>+r & <tab>) and Favorite Characters (<alt>+v & <tab>)
Demise of OpenOffice.org Compose Special Characters extension is NOT OUR BUG
(In reply to najoll from comment #0)
> Apparently an extension provides the functionality. (See
> characters-by-shortcut-keys/) It seems to me a poor show that one should
> have to rely upon an extension.
You don't as noted, but the OpenOffice.org era extension is still available , and its 2.0.10 build remains functional in current LibreOffice master/6.2 builds and earlier.
I am afraid that it is unclear to me that any of the methods listed - save perhaps the (working / available) version of the extension - actually allows one to assign a keyboard shortcut to insert a character, i.e. to press a key together with some modifier key, in order to insert the character. I tried the autotext method but when I selected the 'shortcut' box and press <alt> <p> - the key combination I wished to assign to my text, a strange box about paths appeared. Also, is selected text the text that is being assigned, and is that text meant to appear in the right pane? I select some text, press <ctrl-F3>, and the right pane remains empty. Or is the autotext text what the window calls 'Name'? Really, all of this seems pretty opaque, even after wading through a page (as I did) of the built-in help.
This is invalid, LO provides multiple methods for accomplishing exactly the requested function, or even the external Extension with encoded macros.
There is not reason for LO to implement in core a user defined modifier based (e.g. <Alt> or <Ctl>) shortcut mechanism beyond the provided methods noted comment 1.
Please read the manual ... either AutoText (entering defined "shortcut" + F3), or the AutoCorrect (Replace & With tupple) methods will provide keyboard driven replacement.
 See pages 62-65 of the 5.4 Writer Guide
Or the current online help
Thank you. Once I had read the relevant pages of the manual, I managed to get autotext to work.
However: (1) I regard the procedure as uninituitive and parts of the autocorrect window as confusing; (2) I remain of the view that the autocorrect window, and indeed the manual, should define its terms more than it does; (3) I could not prevent Writer from adding a line break once my autotext was inserted; (4) why not have a 'special characters' menu from within the autotext window?
It seems to me that 1-4 will make people who have come from Ms Word . . inclined to go back to it. I know it has put me off using Writer.