optimize command is deprecated as of 5.5, and a commit in master has already removed it from 5.6. Waiting until 5.6 gives you time to update your build scripts and
composer.json files ahead of the release.
The 5.5 upgrade guide explains the reason for the deprecation:
With recent improvements to PHP op-code caching, the optimize Artisan command is no longer needed. You should remove any references to this command from your deployment scripts as it will be removed in a future release of Laravel.
We’ll remove it in 5.6 and note in the upgrade guide for 5.5. It can break deployment scripts that are calling it if we remove it without warning.
Laravel 5.5 also requires PHP 7, so you should be safe to remove any reference to this command now if you’ve upgraded. Be sure to remove the command from your composer.json
post-update-cmd scripts ahead of the 5.6 release next year.