do any of these native GUI libraries provide just as rich accessibility experiences to disabled people?

If we mean native as in OS-specific GUI APIs built-in to the operating system, then yes. All of the browsers are abstractions on top of that. If we mean cross-platform GUI libraries for Rust I think your choices are like Tauri (which is a webview), Slint, and maybe Fltk with extensions?

Yes, yes they do. WXDragon for Rust, for example, provides quite good accessibility. And maybe accessibility would be better for frameworks like QT or DearImGUI if (1) people didn't act like the web was some paragon of accessibility (hint: it really, really isn't, not when websites misuse web APIs -- which happens quite frequently) and (2) UI framework developers didn't just make a UI framework that looked sexy but left accessibility on the floor.

What's oddly being ignored in this entire thread (so far, as of the time of writing) is that back in 2017-2018 Discord was an Electron app and was completely inaccessible. The web did not magically bring accessibility to the table -- we disabled people had to go yell at Discord to fix their shit, and even then it took them like 2-3 years, and even now Discord has some very very weird accessibility issues that will randomly crop up here and there.