php artisan optimize command was deprecated in Laravel 5.5. Now, after the official release of Laravel 5.6, it is removed.
In Laravel 5.5, use of optimize command in the post-install-cmd and post-update-cmd scripts was no longer available. Now with Laravel 5.6, you cannot use optimize command in your projects.
According to Laravel 5.6 Upgrade guide:
The previously deprecated
optimizeArtisan command has been removed. With recent improvements to PHP itself including the OPcache, theoptimizecommand no longer providers any relevant performance benefit.
In Laravel 5.5 use of optimize command generates a deprecated warning and now after the release Laravel 5.6 it is completely removed. So, if you’re upgrading to Laravel 5.6, be sure to remove this command from composer.json file.
