Actually router_proxy is enabled in config.yml master version.
See https://github.com/ezsystems/ezpublish5/blob/master/ezpublish/config/config.yml#L7

But, according to
https://github.com/symfony/FrameworkBundle/commit/bbede10727830df6bf1e116216183accd4c5f579
that entry has been renamed.

As a result, doing a fresh install produces a error in the configuration step saying

[Symfony\Component\Config\Definition\Exception\InvalidConfigurationException]  
  Unrecognized options "router_proxy" under "framework"