go to stale branch list page then run this script in devtools console 👍 `deleteBranches("org", "repo")`

delete-stale-branches.js

async function deleteBranches(org, repo) {
    await Promise.allSettled(
        Array
        .from(document.getElementsByClassName('js-branch-delete-button'))
        .map((elm) => new Promise((resolve, reject) => {
            const branchName = elm.getAttribute('title').replace('Delete ', '');
            const deleteUrl = `https://github.com/${org}/${repo}/branches/${branchName}`;
            const authenticityToken = elm.closest('form').querySelector('[name="authenticity_token"]').value;

            const method = 'POST';

            const body = new FormData();
            body.append('_method', 'delete');
            body.append('authenticity_token', authenticityToken);

            const headers = headers: { 'x-requested-with': 'XMLHttpRequest' };

            return fetch(deleteUrl, { headers, body, method })
                .then(() => {
                    elm.closest('.Box-row.position-relative').remove();

                    return resolve();
                }).catch(reject);
        }))
    ).then((results) => {
        document.querySelector('.pagination a').click();

        return setTimeout(() => deleteBranches(org, repo), 5000);
    })
}