One of the main new features of WordPress 5.5 is to have XML sitemap, but many websites have used third-party plugins (such as Google XML Sitemaps, yoast SEO, all in one SEO Pack) to generate sitemaps.
In fact, for most sites, it is enough to use the XML site map function, but for some sites with complex content types, or you attach great importance to SEO, the setting options of the third-party site map plugin may be better used.
You can disable XML site maps using the plugin
You can disable WordPress 5.5 + original XML sitemap by installing and activating the WP Disable Sitemap plugin.
Disable XML site map by code
If you don’t want to install the plugin, you can add the following code to the functions.php：
add_filter( 'wp_sitemaps_enabled', '__return_false' );