How can I find out when Terraform will support newly released AWS services?
Terraform is an open source project that is managed on GitHub. You can follow existing issues or open a new one if your enhancement request isn't found. See their issue list at: https://github.com/hashicorp/terraform/issues

As Martin Atkins points out in the comments, the plugin specific features are each developed and tracked in their respective repositories. Here's the one for AWS: https://github.com/terraform-providers/terraform-provider-aws