Namespaced-crates are more solely for clarity as they give an idea of what owns a crate, but still interesting. So reviving this proposal:
I think it fits well because core-crates like amethyst contain re-exports of things like amethyst/ui (accessed as amethyst::ui or amethyst_ui). Also it tries to not break compatibility (it says slashes are ambiguous to feature specifiers, but I never played too much with features in Cargo yet) and doesn't use different entities as namespaces (like user or organization).
The RFC also mentions possible adoption of @. But to me it'd be confusing since the proposal differs from NPM packages where namespaces are users or organizations.