I tried to upgrade from 3.3 to 3.4.x-dev to test compatibility and found that all of my services made private by default. It happes from Symfony\Component\DependencyInjection\Compiler\ResolveChildDefinitionsPass::process method:

foreach ($container->getDefinitions() as $definition) {
    if ($definition->isPrivate()) {
        $definition->setPublic(false);
        $definition->setPrivate(true);
    }
}

->isPrivate() method returns true for all services by default, because \Symfony\Component\DependencyInjection\Definition::$private set as true by default.

/cc @nicolas-grekas