The example in the documentation looks like it uses a repo listing that has been generated where the mercy preview header has been passed in the request.

Please try adding the mercy preview header to your request. I’ve provide an example using curl below:

curl \
     -H "Accept: application/vnd.github.v3+json" \
     -H "Accept: application/vnd.github.mercy-preview+json" \
     -H "Authorization: token ${GITHUB_TOKEN}" \
        https://api.github.com/user/starred

for me it returns the topics field:

    "open_issues_count": 2,
    "license": null,
    "topics": [
      "cats",
      "dogs",
    ],
    "forks": 0,
    "open_issues":