I made no change to my wiki, only updated to 1.35, after noticing that category tree no longer worked I tried a minimum config with only category tree installed. Same result.

Steps to Reproduce:
Minimal config of mediawiki 1.35 with only category tree installed and using default vector skin.

wfLoadExtension('CategoryTree');
$wgCategoryTreeSidebarRoot = 'Category:Support';
$wgCategoryTreeForceHeaders = true;
$wgCategoryTreeSidebarOptions['mode'] = 'pages';
$wgCategoryTreeSidebarOptions['depth'] = 2;

Actual Results:
Category tree does not show in sidebar like in 1.34:

Expected Results: