A secure string is not a valid option to keep secrets out of Terraform configuration files. A secure string is a feature of AWS Systems Manager Parameter Store that allows you to store sensitive data encrypted with a KMS key. However, Terraform does not support secure strings natively and requires a custom data source to retrieve them. The other options are valid ways to keep secrets out of Terraform configuration files. A Terraform provider can expose secrets as data sources that can be referenced in the configuration.
Environment variables can be used to set values for input variables that contain secrets. A -var flag can be used to pass values for input variables that contain secrets from the command line or a file. References =
[AWS Systems Manager Parameter Store], [Terraform AWS Provider Issue #55], [Terraform Providers],
[Terraform Input Variables]

Terraform is built on a plugin-based architecture, enabling developers to extend Terraform by writing new plugins or compiling modified versions of existing plugins1. Terraform plugins are executable binaries written in Go that expose an implementation for a specific service, such as a cloud resource, SaaS platform, or API2.
If there is no existing provider for your API, you can create one using the Terraform Plugin SDK3 or the Terraform Plugin Framework4. References =
*1: Plugin Development - How Terraform Works With Plugins | Terraform | HashiCorp Developer
*2: Lab: Terraform Plug-in Based Architecture - GitHub
*3: Terraform Plugin SDK - Terraform by HashiCorp
*4: HashiCorp Terraform Plugin Framework Now Generally Available

Explanation
HashiCorp Configuration Language (HCL) does not support user-defined functions. You can only use the built-in functions that are provided by the language. The built-in functions allow you to perform various operations and transformations on values within expressions. The general syntax for function calls is a function name followed by comma-separated arguments in parentheses, such as max(5, 12, 9). You can find the documentation for all of the available built-in functions in the Terraform Registry or the Packer Documentation, depending on which tool you are using. References = : Functions - Configuration Language | Terraform : Functions - Configuration Language | Packer

This is a secure way to inject sensitive variables into your Terraform run, as they will not be stored in any file or source code repository. You can also use environment variables or variable files with encryption to pass sensitive variables to Terraform.

Environment variables and connection configurations outside of Terraform are secure options for storing secrets for connecting to a Terraform remote backend. Environment variables can be used to set values for input variables that contain secrets, such as backend access keys or tokens. Terraform will read environment variables that start with TF_VAR_ and match the name of an input variable. For example, if you have an input variable called backend_token, you can set its value with the environment variable TF_VAR_backend_token1.
Connection configurations outside of Terraform are files or scripts that provide credentials or other information for Terraform to connect to a remote backend. For example, you can use a credentials file for the S3 backend2, or a shell script for the HTTP backend3. These files or scripts are not part of the Terraform configuration and can be stored securely in a separate location. The other options are not secure for storing secrets. A variable file is a file that contains values for input variables. Variable files are usually stored in the same directory as the Terraform configuration or in a version control system. This exposes the secrets to anyone who can access the files or the repository. You should not store secrets in variable files1. Inside the backend block within the Terraform configuration is where you specify the type and settings of the remote backend. The backend block is part of the Terraform configuration and is usually stored in a version control system. This exposes the secrets to anyone who can access the configuration or the repository. You should not store secrets in the backend block4. References = [Terraform Input Variables]1, [Backend Type: s3]2,
[Backend Type: http]3, [Backend Configuration]4