Core request handler documentation maybe has conflicting information?
I'm pretty sure the advice stems from this issue: Replace almost all remaining superglobals ($_GET, $_POST, etc.) with Symfony Request object. The opening gambit from that post is:


  In order to depend on using the Request object to determine caching, we need to ensure that everything uses it, rather than raw PHP variables for $_SERVER, $_REQUEST, $_GET, $_POST. 


I don't the quoted comment is suggesting they're deprecated in PHP as such, just that using them in Drupal isn't going to be reliable.