Custom wp-sitemap routes and serve via siteurl value

custom-wp-sitemap-routes.php

<?php
// Disable sitemap stylesheets if needed.
add_filter( 'wp_sitemaps_stylesheet_url', '__return_false' );
add_filter( 'wp_sitemaps_stylesheet_index_url', '__return_false');

add_action( 'init', 'register_custom_sitemap_rewrites', 20 );

/**
 * Register custom `^sitemap/*` rewrite rules for sitemap routes.
 *
 * @return void
 */
function register_custom_sitemap_rewrites(): void {
	// Register index route.
	add_rewrite_rule( '^sitemap/wp-sitemap\.xml$', 'index.php?sitemap=index', 'top' );

	// Register custom rewrites for the XSL stylesheet if needed.
	//add_rewrite_rule( '^sitemap/wp-sitemap\.xsl$', 'index.php?sitemap-stylesheet=sitemap', 'top' );
	//add_rewrite_rule( '^sitemap/wp-sitemap-index\.xsl$', 'index.php?sitemap-stylesheet=index', 'top' );

	// Register routes for providers.
	add_rewrite_rule(
		'^sitemap/wp-sitemap-([a-z]+?)-([a-z\d_-]+?)-(\d+?)\.xml$',
		'index.php?sitemap=$matches[1]&sitemap-subtype=$matches[2]&paged=$matches[3]',
		'top'
	);
	add_rewrite_rule(
		'^sitemap/wp-sitemap-([a-z]+?)-(\d+?)\.xml$',
		'index.php?sitemap=$matches[1]&paged=$matches[2]',
		'top'
	);
}

/**
 * Setting `home` and `siteurl` options to different values helps us set
 * permalinks correctly, but it causes some problems for resources that we still
 * want to serve from WordPress. This ultility function filters those resources to use the home url.
 *
 * @param string $resource_url URL of a WordPress resource.
 *
 * @return string
 */
function update_resource_url( string $resource_url ): string {
	$home_path     = wp_make_link_relative( home_url() );
	$resource_path = wp_make_link_relative( $resource_url );

	if ( ! empty( $home_path ) ) {
		$resource_path = preg_replace( sprintf( '#^%s/*#', $home_path ), '/', $resource_path );
	}

	return home_url( $resource_path );
}

add_filter( 'home_url', 'fix_wp_sitemap_url', 10, 2 );

/**
 * Filters the home URL for any `/wp-sitemap*` requests, as we want these to use
 * the `siteurl` option value instead of `home`. It also prepends `/sitemap` to match
 * our custom rewrite rules.
 *
 * Example request paths:
 * - /sitemap/wp-sitemap.xml
 * - /sitemap/wp-sitemap-posts-post-1.xml
 * - /sitemap/wp-sitemap-news-10.xml
 * - /sitemap/wp-sitemap-posts-topic-1.xml
 *
 * @param string $url  The complete home URL including scheme and path.
 * @param string $path Path relative to the home URL. Blank string if no path is specified.
 *
 * @return string
 */
function fix_wp_sitemap_url( string $url, string $path ): string {
	if ( str_starts_with( $path, '/wp-sitemap' ) ) {
		return site_url( '/sitemap' . $path );
	}

	return $url;
}

add_filter( 'wp_sitemaps_index_entry', 'fix_wp_sitemaps_index_entry_urls', 10, 1 );

/**
 * Filters the sitemap entry for the sitemap index.
 *
 * @uses update_resource_url() to update the URL to use siteurl instead of home URL.
 *
 * @param array $sitemap_entry Sitemap entry for the post.
 */
function fix_wp_sitemaps_index_entry_urls( array $sitemap_entry ): array {
	$sitemap_entry['loc'] = update_resource_url( $sitemap_entry['loc'] );

	return $sitemap_entry;
}