Source

File: wp-includes/sitemaps/class-wp-sitemaps-renderer.php

private function check_for_simple_xml_availability() {
		if ( ! class_exists( 'SimpleXMLElement' ) ) {
			add_filter(
				'wp_die_handler',
				static function () {
					return '_xml_wp_die_handler';
				}
			);

			wp_die(
				sprintf(
					/* translators: %s: SimpleXML */
					esc_xml( __( 'Could not generate XML sitemap due to missing %s extension' ) ),
					'SimpleXML'
				),
				esc_xml( __( 'WordPress › Error' ) ),
				array(
					'response' => 501, // "Not implemented".
				)
			);
		}
	}