Source

File: wp-includes/rest-api/search/class-wp-rest-post-search-handler.php

public function __construct() {
		$this->type = 'post';

		// Support all public post types except attachments.
		$this->subtypes = array_diff(
			array_values(
				get_post_types(
					array(
						'public'       => true,
						'show_in_rest' => true,
					),
					'names'
				)
			),
			array( 'attachment' )
		);
	}