Edit: class-walker-page.php

<?php
/**
 * Post API: Walker_Page class
 *
 * @package WordPress
 * @subpackage Template
 * @since 4.4.0
 */

/**
 * Core walker class used to create an HTML list of pages.
 *
 * @since 2.1.0
 *
 * @see Walker
 */
class Walker_Page extends Walker {

/**
	 * What the class handles.
	 *
	 * @since 2.1.0
	 * @var string
	 *
	 * @see Walker::$tree_type
	 */
	public $tree_type = 'page';

/**
	 * Database fields to use.
	 *
	 * @since 2.1.0
	 * @var string[]
	 *
	 * @see Walker::$db_fields
	 * @todo Decouple this.
	 */
	public $db_fields = array(
		'parent' => 'post_parent',
		'id'     => 'ID',
	);

/**
	 * Outputs the beginning of the current level in the tree before elements are output.
	 *
	 * @since 2.1.0
	 *
	 * @see Walker::start_lvl()
	 *
	 * @param string $output Used to append additional content (passed by reference).
	 * @param int    $depth  Optional. Depth of page. Used for padding. Default 0.
	 * @param array  $args   Optional. Arguments for outputting the next level.
	 *                       Default empty array.
	 */
	public function start_lvl( &$output, $depth = 0, $args = array() ) {
		if ( isset( $args['item_spacing'] ) && 'preserve' === $args['item_spacing'] ) {
			$t = "\t";
			$n = "\n";
		} else {
			$t = '';
			$n = '';
		}
		$indent  = str_repeat( $t, $depth );
		$output .= "{$n}{$indent}<ul class='children'>{$n}";
	}

/**
	 * Outputs the end of the current level in the tree after elements are output.
	 *
	 * @since 2.1.0
	 *
	 * @see Walker::end_lvl()
	 *
	 * @param string $output Used to append additional content (passed by reference).
	 * @param int    $depth  Optional. Depth of page. Used for padding. Default 0.
	 * @param array  $args   Optional. Arguments for outputting the end of the current level.
	 *                       Default empty array.
	 */
	public function end_lvl( &$output, $depth = 0, $args = array() ) {
		if ( isset( $args['item_spacing'] ) && 'preserve' === $args['item_spacing'] ) {
			$t = "\t";
			$n = "\n";
		} else {
			$t = '';
			$n = '';
		}
		$indent  = str_repeat( $t, $depth );
		$output .= "{$indent}</ul>{$n}";
	}

/**
	 * Outputs the beginning of the current element in the tree.
	 *
	 * @see Walker::start_el()
	 * @since 2.1.0
	 * @since 5.9.0 Renamed `$page` to `$data_object` and `$current_page` to `$current_object_id`
	 *              to match parent class for PHP 8 named parameter support.
	 *
	 * @param string  $output            Used to append additional content. Passed by reference.
	 * @param WP_Post $data_object       Page data object.
	 * @param int     $depth             Optional. Depth of page. Used for padding. Default 0.
	 * @param array   $args              Optional. Array of arguments. Default empty array.
	 * @param int     $current_object_id Optional. ID of the current page. Default 0.
	 */
	public function start_el( &$output, $data_object, $depth = 0, $args = array(), $current_object_id = 0 ) {
		// Restores the more descriptive, specific name for use within this method.
		$page = $data_object;

$current_page_id = $current_object_id;

if ( isset( $args['item_spacing'] ) && 'preserve' === $args['item_spacing'] ) {
			$t = "\t";
			$n = "\n";
		} else {
			$t = '';
			$n = '';
		}
		if ( $depth ) {
			$indent = str_repeat( $t, $depth );
		} else {
			$indent = '';
		}

$css_class = array( 'page_item', 'page-item-' . $page->ID );

if ( isset( $args['pages_with_children'][ $page->ID ] ) ) {
			$css_class[] = 'page_item_has_children';
		}

if ( ! empty( $current_page_id ) ) {
			$_current_page = get_post( $current_page_id );

if ( $_current_page && in_array( $page->ID, $_current_page->ancestors, true ) ) {
				$css_class[] = 'current_page_ancestor';
			}

if ( $page->ID === (int) $current_page_id ) {
				$css_class[] = 'current_page_item';
			} elseif ( $_current_page && $page->ID === $_current_page->post_parent ) {
				$css_class[] = 'current_page_parent';
			}
		} elseif ( (int) get_option( 'page_for_posts' ) === $page->ID ) {
			$css_class[] = 'current_page_parent';
		}

/**
		 * Filters the list of CSS classes to include with each page item in the list.
		 *
		 * @since 2.8.0
		 *
		 * @see wp_list_pages()
		 *
		 * @param string[] $css_class       An array of CSS classes to be applied to each list item.
		 * @param WP_Post  $page            Page data object.
		 * @param int      $depth           Depth of page, used for padding.
		 * @param array    $args            An array of arguments.
		 * @param int      $current_page_id ID of the current page.
		 */
		$css_classes = implode( ' ', apply_filters( 'page_css_class', $css_class, $page, $depth, $args, $current_page_id ) );
		$css_classes = $css_classes ? ' class="' . esc_attr( $css_classes ) . '"' : '';

if ( '' === $page->post_title ) {
			/* translators: %d: ID of a post. */
			$page->post_title = sprintf( __( '#%d (no title)' ), $page->ID );
		}

$args['link_before'] = empty( $args['link_before'] ) ? '' : $args['link_before'];
		$args['link_after']  = empty( $args['link_after'] ) ? '' : $args['link_after'];

$atts                 = array();
		$atts['href']         = get_permalink( $page->ID );
		$atts['aria-current'] = ( $page->ID === (int) $current_page_id ) ? 'page' : '';

/**
		 * Filters the HTML attributes applied to a page menu item's anchor element.
		 *
		 * @since 4.8.0
		 *
		 * @param array $atts {
		 *     The HTML attributes applied to the menu item's `<a>` element, empty strings are ignored.
		 *
		 *     @type string $href         The href attribute.
		 *     @type string $aria-current The aria-current attribute.
		 * }
		 * @param WP_Post $page            Page data object.
		 * @param int     $depth           Depth of page, used for padding.
		 * @param array   $args            An array of arguments.
		 * @param int     $current_page_id ID of the current page.
		 */
		$atts = apply_filters( 'page_menu_link_attributes', $atts, $page, $depth, $args, $current_page_id );

$attributes = '';
		foreach ( $atts as $attr => $value ) {
			if ( is_scalar( $value ) && '' !== $value && false !== $value ) {
				$value       = ( 'href' === $attr ) ? esc_url( $value ) : esc_attr( $value );
				$attributes .= ' ' . $attr . '="' . $value . '"';
			}
		}

$output .= $indent . sprintf(
			'<li%s><a%s>%s%s%s</a>',
			$css_classes,
			$attributes,
			$args['link_before'],
			/** This filter is documented in wp-includes/post-template.php */
			apply_filters( 'the_title', $page->post_title, $page->ID ),
			$args['link_after']
		);

if ( ! empty( $args['show_date'] ) ) {
			if ( 'modified' === $args['show_date'] ) {
				$time = $page->post_modified;
			} else {
				$time = $page->post_date;
			}

$date_format = empty( $args['date_format'] ) ? '' : $args['date_format'];
			$output     .= ' ' . mysql2date( $date_format, $time );
		}
	}

/**
	 * Outputs the end of the current element in the tree.
	 *
	 * @since 2.1.0
	 * @since 5.9.0 Renamed `$page` to `$data_object` to match parent class for PHP 8 named parameter support.
	 *
	 * @see Walker::end_el()
	 *
	 * @param string  $output      Used to append additional content. Passed by reference.
	 * @param WP_Post $data_object Page data object. Not used.
	 * @param int     $depth       Optional. Depth of page. Default 0 (unused).
	 * @param array   $args        Optional. Array of arguments. Default empty array.
	 */
	public function end_el( &$output, $data_object, $depth = 0, $args = array() ) {
		if ( isset( $args['item_spacing'] ) && 'preserve' === $args['item_spacing'] ) {
			$t = "\t";
			$n = "\n";
		} else {
			$t = '';
			$n = '';
		}
		$output .= "</li>{$n}";
	}
}

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