TL;DR

There is a new attribute retain_backups_on_delete that allows for deleting a CloudSQL instance without deleting its backups.

It would be great to support this, as it's always concerned me that deleting an instance deletes the backups immediately.

Terraform Resources

https://registry.terraform.io/providers/hashicorp/google/latest/docs/resources/sql_database_instance#retain_backups_on_delete-1