It might be only tangentially related to your point, but there are Slack API-based clients for [emacs][1] and [weechat][2].

So screen-reader usability is still a thing. The fact it's not using a proper standard open protocol is a problem.

[1]: https://github.com/yuya373/emacs-slack

[2]: https://github.com/wee-slack/wee-slack