That’s basically it, I think. I guess it’s possible that PyPI will publish the metadata for sdists as separate files so that consumers can avoid downloading the full sdist, but I’m not sure how important that is (sdists are typically a lot smaller than wheels, aren’t they?)
I don’t know that there’s any sort of “roadmap” as such. It’s more about dependencies - backends can’t produce metadata 2.2 until PyPI allows it to be uploaded, and it’s pointless for installers to add support for it until there’s some on PyPI to justify doing so. Individual projects will work on support once it’s possible for them to do so, and when they have resources to do the work.