Source

File: wp-includes/sitemaps/providers/class-wp-sitemaps-posts.php

public function get_object_subtypes() {
		$post_types = get_post_types( array( 'public' => true ), 'objects' );
		unset( $post_types['attachment'] );

		$post_types = array_filter( $post_types, 'is_post_type_viewable' );

		/**
		 * Filters the list of post object sub types available within the sitemap.
		 *
		 * @since 5.5.0
		 *
		 * @param WP_Post_Type[] $post_types Array of registered post type objects keyed by their name.
		 */
		return apply_filters( 'wp_sitemaps_post_types', $post_types );
	}