Not sure why not, according to the IP space reported by their DNS entries for api(dot)openai(dot)com, they are using cloudflare in the front of their systems. According to cloudflare all they would need to do to support this would be to “flip” this “switch”.
We too could really use an IPv6 address. Reason being, we run services in containers on AWS, and currently we need to use an EC2 instance just to enable NAT, which is cheaper than using a NAT Gateway but still costs us. If we could hit the OpenAI API via IPv6, that would be free, and we could remove that NAT instance.