As this post to Laravel News mentions the upcoming v5.6 release of the framework will remove the
artisan optimize functionality from the project's
artisan command line tool.
The Artisan 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 project has provided some reasoning behind the change, mostly having to do with the overall performance improvements in the PHP language itself. Additionally, there was some discussion around the removal before it was finalized.