On the Symfony blog there's a quick post from Fabien Potencier (a sort of follow-up to this one) that talks about the end of Silex, a popular Symfony-based microframework, now that Symfony 4 and Flex exist.
What about Silex in a Symfony 4 world? During the last few months, and as an exercise when working on Flex, I have migrated several applications from Silex to Symfony 4. And the conclusion is that Symfony 4 feels like using Silex.
Using Symfony 4 and Flex feels as lightweight as using Silex. [...] Moving away from Silex is also made simpler as Symfony 4 almost auto-configure all your services. [...] For all these reasons, I would say that Silex is not needed anymore. So, we've decided to not support Symfony 4 in Silex, or at least not add the new features added in 3.4.
The comments on the post seem mostly supportive of the decision, realizing that what Symfony 4/Flex bring to the table all but replaces Silex anyway. A migration guide is in the works but hasn't been completed yet at the time of this posting (see this issue for the latest updates on that guide).