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As the existing answers are not quite clear about it:

1. this is not an error, only a warning. The abandoned package still exists, and Magento already replaced the dependency somewhere in version 2.1 so you don't have to worry about it.

2. There seem to be cases where the warning prevents further installations. In that case, or if you want to get rid of the warning and use the current version of phpcsfixer (which is a dev tool and not required for Magento to operate), run these commands in your project root:

    composer remove fabpot/php-cs-fixer
    composer require --dev friendsofphp/php-cs-fixer
   

   Do not edit composer.json or even composer.lock directly to avoid conflicts!