I was recently looking for IPv4/IPv6-only HTTP services hosted by major CDNs (for https://github.com/pmarks-net/ipvfoo/issues/60), and found these:

Cloudflare: https://ipv4.icanhazip.com + https://ipv6.icanhazip.com

Akamai: https://ipv4.whatismyip.akamai.com + https://ipv6.whatismyip.akamai.com

The problem with https://checkip.amazonaws.com is that it neither supports IPv6, nor claims that it will remain IPv4-only, so it's not clear what you'll get in the future.