Rest full web service API url is changed in d8
From the related issue:


  Drupal core no longer uses accept headers for routing but rather rely on a _format query parameter, because browser, (reverse) proxy and CDN support is very poor, and hence could lead to severe bugs. See See #2364011: [meta] External caches mix up response formats on URLs where content negotiation is in use for more background.


So it was a conscious decision, to help prevent common bugs out in the wild. I wouldn't advise trying to find a workaround, the motivation for changing it is sound. But if you want/need to, perhaps just a simple rewrite rule that would take requests for *.json and forward them on to *?_format=json would suffice.