According to the July 12 amendment to the blog post https://blog.twitter.com/2016/alt-text-support-for-twitter-cards-and-the-rest-api the
include_ext_alt_text is no longer required since it is enabled by default. I’ve found this to be untrue, it is still required to supply the
include_ext_alt_text param in order to retrieve the alt text.
Tested this with https://twitter.com/todd/status/704722472898334720