Rest full web service API url is changed in d8 I have rest service url for a node in d6  as 

https://192.168.1.236/rest_endpoint/node/1140.json 


but the same url in drupal 8 is not working at all , for d8 i have  to give 

https://192.168.1.236/rest_endpoint/node/1140?_format=json


here 1140 is the node id and 192.168.1.236 is the base url . 

As the url is changing i have to modify many scripts .What may be the reason for this ? or is there any workarounds ?
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.