for completeness, swiftinit.org also hosts package references which overlap with the ones served by swiftpackageindex.com, swift.org, and apple.com. many projects like SwiftNIO and Vapor also host their own package-specific references.
although basic historical archiving is straightforward to acheive, true documentation versioning is extremely difficult to implement when you add URL stability as a requirement.
moreover, certain packages like swift-syntax contain both a large number of symbols, and a large number of tag releases, which makes indexing and serving this kind of content a significant engineering task in and of itself. although most packages do not vend enough API or release often enough to strain the documentation service provider, a handful of “fat” packages like swift-syntax are currently blocking this feature, at least for swiftinit.org.
i believe this was previously discussed at the last meeting of the documentation working group.