Vector provides enhancements to the Vector skin
This extension requires MediaWiki 1.35+.
Example LocalSettings.php additions
wfLoadExtension( 'CollapsibleVector' );
Before configuring this extension, see extension.json and become familiar with the initial state and structure of the $wgCollapsibleVector configuration variable. Essentially it's an array of arrays, keyed by feature name, each containing global and user keys with boolean values. "global" indicates that it should be turned on for everyone always, while user indicates that users should be allowed to turn it on or off in their user preferences.
To enable a preference by default but still allow users to disable it in preferences, use something like...
$wgDefaultUserOptions['collapsiblevector-collapsiblenav'] = 1;