WordPress – How to rewrite the undefined function each() correctly in this case?

Note that each() function has been deprecated as of PHP 7.2.0, and REMOVED as of PHP 8.0.0. Relying on this function is highly discouraged.

Also note that each() function is typically used in conjunction with list() to traverse an array.

So the alternative is to use the foreach() function on the array key / value pairs (but not only the values), to traverse $this->element_activities[ $tag ] array.

So the correct revisited working code should be:

public function callElementActivities( $tag ) {
    do_action( 'vc_mapper_call_activities_before' );

    if ( isset( $this->element_activities[ $tag ] ) ) {
        foreach ( $this->element_activities[ $tag ] as $method => $params ) {
            switch ( $method ) {
                case 'drop_param':
                    WPBMap::dropParam( $params['name'], $params['attribute_name'] );
                    break;
                case 'add_param':
                    WPBMap::addParam( $params['name'], $params['attribute'] );
                    break;
                case 'mutate_param':
                    WPBMap::mutateParam( $params['name'], $params['attribute'] );
                    break;
                case 'drop_shortcode':
                    WPBMap::dropShortcode( $params['name'] );
                    break;
                case 'modify':
                    WPBMap::modify( $params['name'], $params['setting_name'], $params['value'] );
                    break;
            }
        }
    }
}

IT should work now.