The toolkit still seems quite incomplete for desktop use (as opposed to embedded/appliance UIs), as it doesn't support menu bars in an issue open since 2020: https://github.com/slint-ui/slint/issues/38
A comprehensive toolkit like Slint is an enormous and ambitious task. We prioritize feature implementation based on user demand and needs. As of now, we haven't received any specific requests from users for the menu bar feature, but we know it is important (we filled the issue ourselves)
What is a "menu bar?" Something other than a regular application menu?
The toolkit doesn't support regular application menus or right-click menus.