wp-includes/rest-api/class-wp-rest-request.php:237Sets the header on request.
$keystring$valuestring public function set_header( $key, $value ) { $key = $this->canonicalize_header_name( $key ); $value = (array) $value; $this->headers[ $key ] = $value; }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-request.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.