getServerSideProps
  |> getServerSideSitemapLegacy
  |> withXMLResponseLegacy
  |>
    res.setHeader('Content-Type', 'text/xml')
    res.write(content)
    res.end()

https://github.com/iamvishnusankar/next-sitemap/blob/master/packages/next-sitemap/src/ssr/response.ts

SSR에서 XML 형식의 사이트맵을 생성하기.