We should look at removing this from our setup.php:

https://github.com/moodle/moodle/commit/de7e4ac97d2226c4237663e2a9e46d4e3bcc988b

I have been having issues using an external lib that relies on http_build_query() using the default handling of the argument -  &  not   & amp; 

We also tell http_build_query explicitly to use the normal default value of & in many areas of our code eg:
https://github.com/moodle/moodle/blob/87afa4d7595eeafdf9e3f83d53272ee61fb66780/enrol/lti/ims-blti/OAuthBody.php#L104
https://github.com/moodle/moodle/blob/0fd37bf5d8d7e9741f698dd5cb9019717117a7fa/lib/filelib.php#L3862

Owncloud and Drupal have also run into this and removed it from their end - as per:
https://github.com/owncloud/core/issues/14782
https://github.com/owncloud/core/pull/15232

https://www.drupal.org/node/2476237

https://www.drupal.org/node/303154

In Telegram chat - Andrew found this:
https://www.sitepoint.com/php-and-standards-arg-separatoroutput/
(Looks like all of the & have been converted to & in the article making it nonsense -  but - at some point in the past, PHP docs incorrectly stated that & amp; should be set for XHTML compliance.)

We should look at removing this.