wp-includes/query.php:692Is the query for a sitemap?
boolfunction is_sitemap(): bool { global $wp_query; if ( ! isset( $wp_query ) ) { _doing_it_wrong( __FUNCTION__, __( 'Conditional query tags do not work before the query is run. Before then, they always return false.' ), '7.1.0' ); return false; } return $wp_query->is_sitemap();}Introduced in 7.1.0.
Signature, return type and hooks compared across 1 parsed release.
src/wp-includes/query.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.