In principle, multiple help pages could be classified with \docType{package}, not only the PKG-package overview page. For example, I have seen packages that set this type also for PKG-deprecated or PKG-defunct overview pages, justifiably, even if the docType info is currently not used much by R itself ... So I am not sure if it makes sense to both deprecate and expand @docType package towards "_PACKAGE". OTOH, also roxygen2 users can always create additional Rd files directly if needed.

I haven't looked at the internals, but maybe a less disruptive alternative could be for @docType package to only make sure that there is a PKG-package alias if also @name PKG (almost restoring the pre-7.0.0 behavior), and only in this case (where the desired alias was not set) throw a message about the new "_PACKAGE" feature (so people can opt out).

While in principle, there might be multiple topics with \docType{package}, I don't think there's any practical advantage to it? As far as I know there is no special treatment for these topics.