Thanks for the post! Always good to check up on these sorts of things 
I’ve enabled IPv6 for the CloudFront distributions of static.rust-lang.org and www.rust-lang.org (note that rust-lang.org is a DNS-provider-redirect and may not be ipv6 yet).
Unfortunately Heroku, where crates.io is hosted, does not have IPv6 support yet. Additionally, rustup.rs along with domains like doc.rust-lang.org are hosted directly on EC2 which unfortunately doesn’t have direct IPv6 access either.