API v1 has been retired from Twitter, with XML, RSS, and ATOM response formats retiring along with it, leaving only JSON for updated version API v1.1.
Explaining the change, Twitter said: “XML, Atom, and RSS are infrequently used today, and we’ve chosen to throw our support behind the JSON format shared across the platform. Consequently, we’ve decided to discontinue support for these other formats. For historical context, when we originally built the API all major languages did not have performant, well vetted libraries supporting JSON - today they do.”
Twitter also warned that widgets that are based on the Search API and API version 1.0 will be deprecated, and recommends using embedded timelines in its place. It was also said that API V1.1 will be documented as SSL-only.
This is the first major update of the API since its launch, and will allow users to query the API on a per endpoint basis a lot more than they were previously able to.
All applications replicating the core Twitter experience, usually called "clients", will have some new restrictions placed on them, including a 100,000 user token limit, and will require to authenticate all of their requests with OAuth 1.0a or Application-only authentication.