Just dropping to say this is one we as streamers can be addressing without waiting for Twitch to add more features. There's a great website here: https://sites.google.com/view/livestreamcaptions/home which details various methods of adding computer generated captions. The method I use involves a plugin for OBS, and sends captions to Twitch's integrated caption feature, allowing it to be adjusted through settings for that feature & also leaves captions in the VOD. There is also a Closed Captions tag on Twitch to use after setting one of these options up, which I have gotten feedback from deaf/hard of hearing viewers that they search for streams using the tag.

That said: Still up voting this and would encourage others to do the same because having more, higher quality and easier to implement accessibility options would definitely improve things + lead to more streams with captions. Just wanted to put in about the current third party options so people checking this post can find & implement them in the meantime.