Regarding using a second registry as a curation signal: I think we could do a lot better instead with a tags system, where packages can be flagged with various tags at registration time, with implementation to surface them in Pkg.
Why? Let’s think about if we wanted to express more than 1 thing, not just curated vs not, but also say permissively licensed vs copyleft. You could imagine wanting curated permissively licensed packages. But registries only really make sense as unions, not intersections, so to express that we would need 4 registries, one for each leaf combination. Totally unscalable.
Instead, I think this metadata would make more sense as a list of string tags attached to each package. Then Pkg could be taught to handle queries with tags (which packages are available matching such and such tags), and the Preferences system could be used to say I always want all my packages to be resolved from the curated tag, for example.
Also, cargo has a tags system, which is generally a good sign for it being a well-implementable feature, but I haven’t looked in detail how it works or what it affords. Potentially it’s for a different use-case than I have in mind here.