Which method is NOT suitable for deploying custom settings in software development?

The method that is not suitable for deploying custom settings in software development is the .gitignore file. The purpose of the .gitignore file is to specify intentionally untracked files that Git should ignore, and it helps to prevent specific files from being included in version control. This is useful for not committing sensitive data, build artifacts, or environment-specific configurations that should not be shared. However, it does not serve any role in the deployment process or the actual transfer of custom settings code or configurations.

In contrast, environment variables, data templates, and package.xml files are all methods related to the deployment process. Environment variables can be used to configure application settings dynamically, data templates can ensure that custom settings are correctly structured and imported, and package.xml files specifically define the components that will be included in a deployment package when working with metadata in systems like Salesforce, facilitating the deployment of configurations and custom settings effectively.