On MW 1.24 Vector skin, with the user preference ticked or unticked, the sidebar still has items collapsible. The $wgVectorFeatures set to true or false only makes the option come and go from preferences, not enable or disable the collapsible items in the sidebar.
Topic on Extension talk:CollapsibleVector
Hi thanks for reporting this. I have reported it https://phabricator.wikimedia.org/T76314 here.