Twitter has added an alt text capability to iOS and Android apps posting images in tweets which it says are to improve accessibility to everyone.

To ensure publishers and third-party clients also have the capability to add alt text to images, we've extended our platform products to both the REST API and Twitter Cards. We know this is especially important for specialized Twitter clients for the visually impaired such as EasyChirp, Chicken Nugget, and The Qube. Twitter Add Alt Text Facility To Images in Tweets [blog.twitter.com]