On the Symfony blog there's a post with a major announcement for the users of the framework - the end of the Symfony Standard Edition, a release version that tried to bundle in the common tools a developer might need.

When I started to work on Symfony 2 many moons ago, I decided to create a set of decoupled components instead of a monolith framework like symfony 1. This choice was probably one of the key reasons of the immediate success of Symfony 2. [...] Of course, people don't want to assemble the components themselves when starting a new project. To fill the gap, we created the Flex was born. [...] Thanks to the new Flex approach, we've decided to not provide the Symfony Standard Edition for Symfony 4.0.

In the post Fabien talks about some of the history behind the Standard Edition including why it was made and what problems it solved. He then points out that, with the introduction of Flex, many of the issues and flexibility the SE lacked are worked out. While it is a little more complex to get started with a Flex application, the unpacking functionality helps with bringing in bundles of packages related packages without having to require each individually.