Protected branches are available in public repositories with GitHub Free and GitHub Free for organizations, and in public and private repositories with GitHub Pro, GitHub Team, GitHub Enterprise Cloud, and GitHub Enterprise Server. For more information, see GitHub's products in the GitHub Help documentation.

When authenticated with admin or owner permissions to the repository, you can use this endpoint to check whether a branch requires signed commits. An enabled status of true indicates you must sign commits on this branch. For more information, see Signing commits with GPG in GitHub Help.

Note: You must enable branch protection to require signed commits.
Get commit signature protection