A stylesheet function that is called from no expression anywhere in the corpus — neither in its own stylesheet nor in one that imports it — is dead code and should be removed. Calling itself does not count, so a recursive function used nowhere else is still flagged.
Incorrect:
<xsl:function name="my:format">
<xsl:param name="value"/>
<xsl:value-of select="$value"/>
</xsl:function>
<!-- my:format is never referenced in match or select -->
Correct:
<xsl:function name="my:format">
<xsl:param name="value"/>
<xsl:value-of select="$value"/>
</xsl:function>
<xsl:template match="/">
<p><xsl:value-of select="my:format(title)"/></p>
</xsl:template>