wp-includes/rest-api/class-wp-rest-response.php:214Checks if the response is an error, i.e. >= 400 response code.
bool public function is_error() { return $this->get_status() >= 400; }Introduced in 4.4.0. Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-includes/rest-api/class-wp-rest-response.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.