Source

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

protected function check_is_post_type_allowed( $post_type ) {
		if ( ! is_object( $post_type ) ) {
			$post_type = get_post_type_object( $post_type );
		}

		if ( ! empty( $post_type ) && ! empty( $post_type->show_in_rest ) ) {
			return true;
		}

		return false;
	}