With the impending shutdown of ClientLogin on April 20, Google has decided to shut down the old version of the Webmaster Tools API on the same day. This will require developers to switch over to the new API by that time.
The Google Webmaster Tools API was updated to the new version last year. It's not clear if everyone using the old API has moved to the new version since it did not contain an important feature of getting keyword data from Web content, and several online tools are dependent on that. But it is time to take quick action now.
The new API has examples in Python, Java and, for those who prefer the command line, there is OACurl. For developers who require programmatic access to search query data for their sites in Webmaster Tools, an open source Python Script is available for now. This is likely to eventually find its way into the API, but there is no confirmed date.
For full details, check out the official blog announcement. If you have any questions on this topic, Google has encouraged users to ask them on the Webmaster Help Forum. To check out the features available in the new version, visit the new API version announcement post.