Setting html_root_url is still immensely useful when building crate documentation locally. If building with cargo doc --no-deps types are not linked unless the source crate specifies html_root_url.
Possible solutions like -Zrustdoc-map are still unstable. Until then, keeping the attribute still is useful and crates like version-sync ensures that all version numbers are kept in sync. After all, there might be more places with version duplication.
It would be nice if usage for docs.rs could be deprecated/made unnecessary, but it still has use for when documentation is special built not on docsrs (say, for exotic platform(s), or closed-source internal registries.)