See Issue 4579
: Special Character Shortcuts
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I have no idea what your situation is, but shortcut keys for special characters are often not as useful as people think. If you have a lot of special characters to type, as when typing in some special language, it would be better to configure an alternate keyboard map, and/or an input method. All modern systems provide those, and OOo doesn't care how the characters are typed.
If you only need a few special symbols, then it may not be worth configuring and remembering special keys for them. You can just use the Insert > Special Character dialog, or define replacements for them that are easier to remember. E.g. by default, Writer will insert the copyright symbol when you type (c)
Writer includes a lot of replacements for common symbols, you can see the list under Tools > AutoCorrect > Replace.
Of course there is a "sweet spot" where you have a few symbols that you use very often, so I'm not saying the feature wouldn't be useful, only that there are alternatives for the most common situations.