So just to clear some things up - the changes to SwiftPM for it to be able to talk to a package registry are available in 5.7 The original proposal was accepted and there has been recent work in SwiftPM to expose this in 5.7 (e.g. expose swift-package-registry as top level tool by tomerd · Pull Request #5679 · apple/swift-package-manager · GitHub).
The other side of it is an actual registry. I have no idea what the status (if any) of the GitHub registry is, but as @hassila pointed out there's an implementation for Artifactory, which is primarily aimed at enterprise in order to put packages behind a firewall. I'm sure others will spin up as well