Resource forks are accessible using the (deprecated) Carbon APIs, but the Rez/DeRez applications are far less convenient than ResEdit. I think Apple should've ported ResEdit to Carbon and included it with the developer tools alongside Rez/DeRez, but it's far too late to be complaining about that.What would be the use(s) of ResEdit in macOS?
I'm not sure what (if any) APIs exist in the Cocoa framework for accessing resource forks.
From what I've been able to find, only the deprecated Resource Manager can access the resource fork the way it was intended - as a database-like collection of tagged resources. Without that manager, apps can only open the resource fork as an opaque block of data - good enough to duplicate files, but not meaningful if you actually want to use the resource fork as it was intended, which makes sense, since the NeXT/Cocoa approach to application design involves storing resources as separate files in the application's bundle, not as resources in the resource fork.
Personally, I think there is still a use for a concept like a resource fork, but these days, it's probably easier to implement the concept using some kind of database file (maybe SQLite) stored in an application's bundle.