How to return all aliases for a path?
The answer is the same as in Drupal 7, because there is no API to return multiple aliases for a single source path, so your only option, if you really need that (I guess there is a reason that you are not using redirects), is a custom query, which is the same as in 7.x, with the only difference that it has a leading /.

So, copied from the other issue:

$aliases = \Drupal::database()-&gt;query('
  SELECT alias
  FROM {url_alias}
  WHERE source = :source
', array(':source' =&gt; '/node/' . $nid))-&gt;fetchAll();