Still no released support for screen readers [1]. The developer seems confused about whether muPDF should have the support [2] (accessibility is the responsibility of the GUI - it needs to provide an accessibility tree for any manually rendered text).
"SumatraPeter" is just a user with "Sumatra" in the name not the developer. The actual developer's comment is:
> For standard windows controls (like buttons or menus) Windows provides the necessary code to support accessibility features used by screen readers etc. Everything else (like PDF text rendered on screen) needs custom code. This code is not easy to write and there is a lot of other code that needs to be written.
Based on your first link it seems they started working on support as described and could use some help finishing it.
[1] https://www.sumatrapdfreader.org/docs/Accessibility-and-Text... [2] https://github.com/sumatrapdfreader/sumatrapdf/issues/321