Beta 11 --> Beta 12: _format property in route definition

Migrating to Beta 12, some ajax functionality was not functioning as expected. The log messages reported an exception,

Symfony\Component\HttpKernel\Exception\NotAcceptableHttpException: No route found for the specified format

Using a debugger to step through the function that throws the exception, I made guesses as to what was needed. Eventually, I stumbled on a fix.

In the .routing.yml file I changed the relevant route definition by removing the _format property as follows.

Before:

ajax.enable:
  path: /ajax/optimizely
  defaults:
    _controller: \Drupal\optimizely\AjaxEnable::enableDisable
    _title: Optimizely Administer AJAX
  requirements:
    _permission: administer optimizely
    _format: json

After:

ajax.enable:
  path: /ajax/optimizely
  defaults:
    _controller: \Drupal\optimizely\AjaxEnable::enableDisable
    _title: Optimizely Administer AJAX
  requirements:
    _permission: administer optimizely

Unfortunately, searching through the Drupal and the Symfony docs, I have not found anything to help me understand why this change to the route definition makes a critical difference.

From the module's point of view, it looks as though the property is actually not needed. The client-side JavaScript that makes the request to the server sets json as the dataType. And the server-side code always instantiates a JsonResponse object to pass back.