Symfony 4.0 has been released together with PHP 7.2 last month (see #9009 for PHP 7.2 support status), so here's the plan:

Sylius 1.0 won't officially support Symfony 4.0 - it will stay with Symfony 3.3-3.4. The reasoning behind this is that we can only rely on Symfony 3.3 features there, so that if something has got deprecated in Symfony 3.4 and there was a forward compatibility layer provided for that, then we still can't use that without breaking backwards compatibility with Symfony 3.3. And some of our components / bundles requires ^3.2 Symfony components.

Sylius 1.1 will require Symfony 3.4 - support for Symfony 4.0 will come as soon as we solve all the deprecations in our and vendors' code.

The only missing step to start fighting deprecations is to set up Travis to include Symfony 4.0 jobs, which I'll have taken care of in this year :)