Source

File: wp-includes/rest-api/endpoints/class-wp-rest-settings-controller.php

protected function set_additional_properties_to_false( $schema ) {
		switch ( $schema['type'] ) {
			case 'object':
				foreach ( $schema['properties'] as $key => $child_schema ) {
					$schema['properties'][ $key ] = $this->set_additional_properties_to_false( $child_schema );
				}

				$schema['additionalProperties'] = false;
				break;
			case 'array':
				$schema['items'] = $this->set_additional_properties_to_false( $schema['items'] );
				break;
		}

		return $schema;
	}