with:
$wgCategoryTreeSidebarRoot = "categories";
in LocalSettings.php, categorytree does not show up in the sidebar. It should, however, as advertised here: https://www.mediawiki.org/wiki/Extension:CategoryTree#Using_CategoryTree_in_the_Sidebar . It does work fine with the timeless skin.
Adding $wgCategoryTreeForceHeaders = true; does not help. mediawiki is logging errors such as this:
Got error 'PHP message: PHP Deprecated: Use of `content` field in portal categorytree-portlet must be array.Previously it could be a string but this is no longer supported. was deprecated in MediaWiki 1.35.0. [Called from VectorTemplate::getSkinData in /var/www/wiki/skins/Vector/includes/VectorTemplate.php at line 167] in /var/www/wiki/includes/debug/MWDebug.php on line 430', referer: https://whatever.wiki/index.php?title=Category:It