> because AAAA records scare the olds

No. They break existing customer expectations.

There are heaps of dualstack API endpoints https://docs.aws.amazon.com/general/latest/gr/rande.html#dua... if that's what the client wants.

The amazonaws.com domain endpoints did not introduce ipv6/AAAA directly is (mostly) due to access control. For better or worse there are a lot of "v4 centric" IAM statements, like aws:SourceIp, in identity/resource/bucket policies. Introducing a new v6 value is going to break all of those existing policies with either unexpected DENYs or, worse, ALLOWs. Thats a pretty poor customer experience to unexpectedly break your existing infrastructure or compromise your access control intentions.

AWS _could_ have audited every potential IAM policy and run a MASSIVE outreach campaign, but something as simple as increasing (opaque!) instance ID length was a multi year effort. And introducing backwards compatibility on a _per policy_ basis is its own infinite security & UX yak shaving exercise as well.

So thats why you have opt-in usage of v6/dualstack in the client/SDK/endpoint name.