Blue/green deployment - aws_db_instance - postgres

I need to upgrade some existing AWS RDS instances (postgres) that were created using aws_db_instance.

I have attempted to update our code. The following has been introduced to an already working resource definition.

blue_green_update {
  enabled = true
}

When I run terraform validate all seems fine…

terraform validate
Success! The configuration is valid.

However, when I attempt to run terraform plan. I run into the following errors:

Planning failed. Terraform encountered an error while generating this plan.

╷
│ Error: 1 error occurred:
│ 	* "blue_green_update.enabled" cannot be set when "engine" is "postgres".
│
│
│
│   with module.rds.aws_db_instance.instances["XXX"],
│   on ../../modules/rds/main.tf line 12, in resource "aws_db_instance" "instances":
│   12: resource "aws_db_instance" "instances" {
│
╵

What am I missing? Any ideas why I can not use blue_green_update.enabled, when the engine is set to: postgres.

After a little more research. I have a feeling the answer might be this is not supported, but I would be interested to understand if I am making a rash assessment here.