How can I find out when Terraform will support newly released AWS services? For example, AWS announced their new "RDS Proxy" service at re:Invent 2019 and it is now in preview. How can I find out when/if Terraform will support this (for planning purposes)?

https://aws.amazon.com/about-aws/whats-new/2019/12/amazon-rds-proxy-available-in-preview/
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