Symfony Flex: What does symfony.lock file do?
It is the proper lock file for Symfony recipes instead of trying to guess via the state of composer.lock
Flex keeps tracks of the recipes it installed in the
symfony.lockfile, which must be committed to your code repository (CVS). https://symfony.com/doc/3.4/setup/flex#how-does-flex-work
It was introduced since Flex 1.0.34 version, originally to solve bugs like this: "Configuring recipes executed twice".