(Disclaimer: I work on Flutter.) As skybrian says, accessibility support is work-in-progress. But if you try an existing Flutter app (e.g. the official Hamilton App) you'll note that most aspects of the app are accessible to VoiceOver, just not quite everything yet. We're actively working on improving our iOS and Android accessibility integration.
If you wrap native widgets it is.
I don't think this is what they are doing, which is what I meant. How are Flutter widgets drawn on the screen?
(disclaimer: I work on Flutter.) Flutter uses its own renderer based on Skia. Flutter essentially asks the OS for an surface to draw onto, and then Flutter draws all the pixels through Skia.
In such case, this is what I feared; I don’t think it is possible to implement proper VoiceOver support with the current public API.
You should file bug reports with Apple to enable that.