I wonder how long it will take for CDK to support this stuff. I'm not sure how they will back themselves out of the corner of subnets having a required ipv4 CIDR property.
Anyone with typescript code that references the non-optional field will get type errors if it is made optional. Though stuffing the string with the v6 subnet wouldn't be the worst thing in the world. Any code that is blindly using it expecting v4 addresses will already be broken if a v6 only subnet is passed in.