I doubt it. They still default to giving an EC2 machine a public IPv4; if their primary motivation would be saving IP costs, they could easily go for the low-hanging fruit.
EC2 is not really low hanging fruit. Most expect to get a ipv4 when creating an instance and many relies on that. Cheaper instances with ipv6 only is probably coming
EC2 is their oldest service; I bet it has oodles of legacy workflows both on Amazon's side and on customers' side. Lambda is likely a much lower hanging fruit in terms of the effort required to make changes to it.
To be pedantic, SNS is their oldest service, if I remember my AWS training correctly.
You are thinking of SQS, the preview version of which was the first to be introduced. S3 was the first non-preview service made generally available.
Correct - you know your AWS launch history!
The number of deployed lambdas likely exceeds the number of ec2 instances by many multiples (pure conjecture). It's a perfect use-case for v6 - dual stack in front and then v6 internally.