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This adds a path parameter and adjusts the query being made on the request accordingly:

add_filter('rest_page_query', function ($args, $request){
  $path = $request->get_param('path');

  if (!empty($path)) {
    $pageByPath = get_page_by_path($path, OBJECT, 'page');

    // overwrite the page id with the page id by path
    $args['p'] = $pageByPath->ID;
  }

  return $args;
}, 10, 2);

Allowing you to request a specific sub page like this:

.../wp-json/wp/v2/pages?path=/pageslug/subpageslug/