tl;dr: We are no longer recommending the AJAX crawling proposal we made back in 2009.
Since the assumptions for our 2009 proposal are no longer valid, we recommend following the principles of progressive enhancement. For example, you can use the History API pushState() to ensure accessibility for a wider range of browsers (and our systems).
Questions and answers
Q: My site currently follows your recommendation and supports _escaped_fragment_. Would my site stop getting indexed now that you've deprecated your recommendation?
A: No, the site would still be indexed. In general, however, we recommend you implement industry best practices when you're making the next update for your site. Instead of the _escaped_fragment_ URLs, we'll generally crawl, render, and index the #! URLs.
Q: Is moving away from the AJAX crawling proposal to industry best practices considered a site move? Do I need to implement redirects?
A: If your current setup is working fine, you should not have to immediately change anything. If you're building a new site or restructuring an already existing site, simply avoid introducing _escaped_fragment_ urls. .
A: In general, websites shouldn't pre-render pages only for Google -- we expect that you might pre-render pages for performance benefits for users and that you would follow progressive enhancement guidelines. If you pre-render pages, make sure that the content served to Googlebot matches the user's experience, both how it looks and how it interacts. Serving Googlebot different content than a normal user would see is considered cloaking, and would be against our Webmaster Guidelines.
If you have any questions, feel free to post them here, or in the webmaster help forum.