There is an /org.openoffice.Office/Substitution configuration tree (defined in officecfg/registry/schema/org/openoffice/Office/Substitution.xcs) offering a "SharePoints" set of variable replacements, and the implementation of the com.sun.star.util.PathSubstitution UNO service (in framework/source/services/substitutepathvars.cxx) is designed to support those "SharePoints" variable replacements (though in a broken way).

However:

For one, there is no sign in the git logs that that "SharePoints" configuration set was ever non-empty, in neither LO nor OOo times.

For another, the PathSubstitution UNO service intends to support those "SharePoints" variable replacements both when resolving strings containing such variable names (substituteVariables, getSubstituteVariableValue) and when introducing variable names into strings (reSubstituteVariables).  However, the latter is implemented (ever since <https://cgit.freedesktop.org/libreoffice/core/commit/?id=a56ebe278ae13de0a2baa821e30c16b4f06a4191> "#100825# New services for path settings and path substitution", in SubstitutePathVariables::impl_reSubstituteVariables, framework/source/services/substitutepathvars.cxx) in such a way that it tries to introduce "$(var-name)" as a replacement for "$(var-name)" (i.e., as a replacement for the variable's name, instead of the variable's content), so will never introduce such a variable name into a string.  And when such a variable name is never introduced into a string via reSubstituteVariables, it's unlikely that it will ever appear in a string passed to substituteVariables or getSubstituteVariableValue (given that the normal use of this UNO service is to introduce variable names when saving a document, and then converting back when loading the document).

In short, this feature looks both unused and broken "since forever".  (It may have been used in some way in StarOffice-only code.)  Theoretically, an extension or external application could populate that "SharePoints" configuration set and cause strings passed to the UNO services substituteVariables or getSubstituteVariableValue methods to contain matching variable names, but that's probably unlikely.

(For completeness, a way to try this out is to place a share/registry/data/Substitution.xcu file into a LO installation with content

> <?xml version="1.0" encoding="UTF-8"?>
> <oor:component-data oor:name="Substitution" oor:package="org.openoffice.Office"
>   xmlns:oor="http://openoffice.org/2001/registry">
>  <node oor:name="SharePoints">
>   <node oor:name="test" oor:op="fuse">
>    <node oor:name="item" oor:op="replace">
>     <prop oor:name="Directory"><value>file:///dummy</value></prop>
>     <node oor:name="Environment">
>      <prop oor:name="Host"><value>*</value></prop>
>     </node>
>    </node>
>   </node>
>  </node>
> </oor:component-data>

If you replace oor:name="test" with oor:name="inst" and try to save an Impress document, it will cause an endless loop in SubstitutePathVariables::impl_reSubstituteVariables (framework/source/services/substitutepathvars.cxx), as it will keep replacing "$(inst)" with "$(inst)" in the (already partly re-substituted) URL <$(inst)/share/palette%3B$(user)/config/standard.sob>.)