I think it's interesting that we take it for granted that accessibility is primarily the responsibility of GUI toolkit developers. I would think that maybe some amount of blame would fall on OS vendors, who have billions of dollars flowing through their organizations, rather than some random open source projects.

If your app can be described to a screen reader, then there should be an easy cross platform API[0] for GUI toolkit makers to use, provided by the underlying platform in a reasonably simple way.

[0]: I am aware of AccessKit. Where's the OS backing?