GtkDialog was deprecated because other changes made it now trivial for any app to re-implement dialogs with just a few CSS classes, and it's more flexible that way too.
>a Rust-native UI toolkit library that does its own drawing, but can consume GTK3 themes
This is never going to happen unless you re-implement nearly all of GTK3 in Rust with the exact same widget hierarchies and all the quirks of its (outdated) rendering model. Why would you want this anyway? GTK themes aren't particularly special, it's just CSS.
Where did you hear this? This is misinformation, menu bars aren't going away. The API is just changed in GTK4: https://docs.gtk.org/gtk4/migrating-3to4.html#gtkmenu-gtkmen...
>GtkDialog as a notable example
GtkDialog was deprecated because other changes made it now trivial for any app to re-implement dialogs with just a few CSS classes, and it's more flexible that way too.
>a Rust-native UI toolkit library that does its own drawing, but can consume GTK3 themes
This is never going to happen unless you re-implement nearly all of GTK3 in Rust with the exact same widget hierarchies and all the quirks of its (outdated) rendering model. Why would you want this anyway? GTK themes aren't particularly special, it's just CSS.