Fragment in my redirects.liquid:

  {% for post in collections.redirected -%}
    {%- for path in post.data.oldPermalinks -%}
      rewrite ^/{{ path }} {{ post.page.url }} permanent;
    {%- endfor %}
  {% endfor %}

Collection in my eleventy.config.js:

  eleventyConfig.addCollection("redirected", function (collectionsApi) {
    return collectionsApi
      .getAll()
      .filter(function (item) {
        return "oldPermalinks" in item.data;
      })
      .sort(function (a, b) {
        const dateDiff = a.date - b.date;
        return dateDiff !== 0
          ? dateDiff
          : a.inputPath.localeCompare(b.inputPath);
      });
  });