see also here: https://phabricator.wikimedia.org/T278306 , tl;dr: sidebar integration is broken w/ vector shipping in 1.35.1

fwiw: it works as expected - aka: the category tree shows up in the sidebar - when using vector from mw 1.34 (which you can get from here: https://gerrit.wikimedia.org/r/plugins/gitiles/mediawiki/skins/Vector/+/refs/heads/REL1_34 ) with mw 1.35.1. Needless to say, this is an evil hack. However, a core extension and the default skin not doing the right thing isn't really what you would expect, either …