WordPress Left Hand Side Admin Menu Always Collapsed

This will properly override the user settings to keep the menu collapsed for all users:

/**
 * Reset user setting to always collapse the admin menu.
 *
 * @see set_user_setting()
 */
function wpdocs_always_collapse_menu() {
    if ( 'f' != get_user_setting( 'mfold' ) ) {
        set_user_setting( 'mfold', 'f' );
    }
}
add_action( 'admin_head', 'wpdocs_always_collapse_menu' );