Compose Desktop is not (screenreader) accessible at all
At least on macOS, the Compose GUI is widely inaccessible for screenreaders. As a VoiceOver user, I even cannot register or login with my Matrix account.. Actually, with Compose we have the same problem as with Java GUIs in their early stages. I can do more research on Compose A11Y on different platforms and screenreaders, if this will be useful.
If Tammy is meant for branding in german public organizations and e-government context, this might result in legal problems, because such organizations are required to offer accessible workplaces (keywords: bGB, BITV). Web-based desktop apps are often used to circumvent this problem, but these are a typical case of „The good is the enemy of the best.“
JavaFX is widely accessible, btw, but it requires different viewmodel implementations due to their reactivity solution..
I don't think Google will put much effort into this due to ignorance, but maybe Jetbrains if they get many requests pushing up the topic. Jetbrains made their IDEs widely screenreader accessible.