I tested this out, and it doesn't work in Drupal 8, unfortunately.

    mymodule.test:
      path: '/mymodule/{testparam}'
      defaults:
        _controller: '\Drupal\mymodule\Controller\TestController::build'
        _title: 'Variable Parameter'
      requirements:
        _permission: 'access content'
        testparam: .+

It picks up the route with "/mymodule/hello", but not "/mymodule/hello/world".