With symfony/symfony#26085 coming with Symfony 4.1 the doc should not mention the deprecated syntax anymore. Most examples in the doc already use the FQCN::method syntax which is good. The parts I've found that should be updated are

• http://symfony.com/doc/current/routing.html#controller-naming-pattern

If your controller is registered as a service, you can also use just one colon separator (e.g. service_name:index).

• http://symfony.com/doc/current/controller/service.html#referencing-your-service-from-routing