Adding a class to "body"

It's a simple way to add a class based on the URL, Drupal 9.
No need to enable the Modules.

/**
* Implements hook_preprocess_html().
*/
function THEME_NAME_preprocess_html(&$variables) {
// Get the current path
$current_path = \Drupal::service('path.current')->getPath();
$internal_path = \Drupal::service('path_alias.manager')->getAliasByPath($current_path);

// Assign it to body class
$variables['attributes']['class'][] = str_replace("/", "", $internal_path);
}

Refer:

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