Beyond that, it was raised that the other options (ssh-like presentation, or requiring a specific user) could actually also assist in the case of multiple indexes.
flat string prefixes which pare part of the package name can’t do this. I can’t even think of a way where tooling can reconcile this for the user due to that you want paying corporations to be able to have these grants be secret on top of it. If the list of prefixes were public, at least here a tool could query it in the multi-index situation and detect: “hey, one of these indexes provided a package that another index had a namespace for”