Get Draft post for non login user as well via WP REST API

functions.php

/**
 * Filter to alter the rest_query args to add draft and filter
 *
 * @param array           $args    Array of arguments for WP_Query.
 * @param WP_REST_Request $request The REST API request.
 *
 * @return array           $args    Array of arguments for WP_Query.
 */
function lubus_custom_allow_drafts_in_rest_query( $args, $request ) {

	if ( ! is_user_logged_in() ) {

		if ( isset( $request['slug'] ) ) {

			$args['post_status'] = array( 'publish', 'draft' );

			add_filter( 'the_posts', 'lubus_update_post_status_in_query_result', 10001, 2 );

			add_filter( 'the_posts', 'lubus_remove_update_post_status_in_query_result', 10002 );
		}
	}
	return $args;
}

add_filter( 'rest_testimonial_query', 'lubus_custom_allow_drafts_in_rest_query', 10, 2 );


/**
 * Filter to update the post status from draft to publish
 *
 * @param WP_Post[] $posts Array of post objects.
 * @param WP_Query  $query The WP_Query instance (passed by reference).
 *
 * @return WP_Post[] $posts Array of post objects.
 */
function lubus_update_post_status_in_query_result( $posts, $query ) {

	// Check if the query is for the REST API and includes a draft post.
	if ( isset( $query->query_vars['post_status'] ) && in_array( 'draft', $query->query_vars['post_status'] ) ) {

		foreach ( $posts as $post ) {

			if ( 'draft' === $post->post_status ) {
				// Change the post status value (for example, to 'publish').
				$post->post_status = 'publish';
			}
		}
	}
	return $posts;
}

/**
 * We are using this function only to remove the filter with functions name update_post_status_in_query_result
 *
 * @param WP_Post[] $posts Array of post objects.
 *
 * @return WP_Post[] $posts Array of post objects.
 */
function lubus_remove_update_post_status_in_query_result( $posts ) {
	remove_filter( 'the_posts', 'lubus_update_post_status_in_query_result', 10001 );
	return $posts;
}