Symfony 4 is a small revolution in the Symfony world. Thanks to Flex, a lot of Symfony tools are becoming obsolete as they are just not needed anymore. The Symfony Installer is one of those tools.
The Symfony Installer is the best way to create a new project with Symfony 2 and Symfony 3. But as of Symfony 4, you can use Composer, plain and simple. It's more standard, more ubiquitous, and that's one less Symfony specific tool to learn.
Like other announcements we have made recently, this blog post officially announces that the Symfony Installer won't be developed anymore. Bugs will still be fixed and its end of life is scheduled for November 2020 as this is when Symfony 3.4 will not be supported anymore.
Composer for the win!