The charge started 1 Feb 2024. IPv6 for the RDS service API endpoints landed 30 Jan 2026. So for two years you could pay the IPv4 bill without having the option to stop.
We run IPv6-first EKS and still can't get to zero. There's no such thing as an IPv6-only EKS cluster, every subnet you give EKS still needs an IPv4 CIDR, and one RDS instance for an internal tool pins IPv4 onto its subnets regardless.
Though ECR, CloudFront origins, public RDS and the RDS service APIs all got IPv6 between April 2025 and January 2026, all after the charge. Wrong order, but it might be what moved them.