WordPress + Widget Context -> For use with a widget with a menu, to show the widget only on pages in that menu.

widget-context-menu-widget.php

<?php

add_filter( 'widget_contexts', function ( $contexts ) {
	$contexts['menu_content'] = [
		'label'       => 'Menu-widget content',
		'description' => 'Show only on pages that have its url/page/post listed in a menu',
		'weight'      => 11,
	];
	return $contexts;
}, 10, 2 );

add_filter( 'widget_context_control-menu_content', function ( $control_args ) {
	$wc = WidgetContext::instance();
	$menus = wp_get_nav_menus();

	$out = [];
	$out[] = '<style>.context-group-menu_content .context-group-wrap { max-height:10em; overflow:auto; padding:0.5em; margin-bottom:0.5em; background:#fff; border:1px solid #ddd; }
.context-group-menu_content label { display:block; padding:0.25em 0; }
</style>';
	foreach($menus as $menu) {
		$out[] = $wc->make_simple_checkbox( $control_args, 'menu-widget-filter-enabled-' . $menu->term_id, $menu->name );
	}
	return implode('', $out);
} );

add_filter( 'widget_context_check-menu_content', function ( $allowed, $options ) {
	global $post;
	$post_id       = (int) $post->ID;
	$post_url      = get_permalink( $post_id );
	$post_url_path = wp_parse_url( $post_url, PHP_URL_PATH );

	if ( ! $post || ! $post instanceof WP_Post || ! $post->ID ) {
		return $allowed;
	}

	foreach ( $options as $option_name => $option_value ) {
		if ( ! $option_value ) {
			continue;
		}

		preg_match( '/menu-widget-filter-enabled-(\d+)/', $option_name, $m );
		if ( ! $m ) {
			continue;
		}

		$menu_id = $m[1] ?? false;
		if ( ! $menu_id ) {
			continue;
		}

		$items = wp_get_nav_menu_items( $menu_id );

		foreach ( $items as $item ) {
			// pages and posts
			if ( ( $post->post_type === $item->object || 'page' === $item->object ) && (int) $item->object_id === $post_id ) {
				return true;
			}
			// custom links
			if ( 'custom' === $item->object ) {
				$custom_link_path = wp_parse_url( $item->url, PHP_URL_PATH );
				if ( $post_url_path === $custom_link_path ) {
					return true;
				}
			}
		}
	}

	return $allowed;
}, 10, 3 );