Eleventy: Setting the permalink in the directory data

The permalink for all templates in a directory can be set in a template directory data file. This can be used along with data variables to generate the ideal URLs.

For example, if you have pages in a posts subdirectory, but you want those pages to be at the root of the generated site, you might use something like this:

File: posts/posts.json

{
  "permalink": "/.html"
}

Source: GitHub thread