You would need to look in the comments for the commit. They are archived here
https://developer.joomla.org/joomlacode-archive/tracker-32.html

I did take a look but didn't find anything relevant. This personally feels like one of those bugs that was carried over without anyone noticing :) because:

• JFormValidator doesn't accept test@joomla.org.:
  https://github.com/joomla/joomla-cms/blob/staging/media/system/js/validate-uncompressed.js#L213-L217
  eg. try to create a new Joomla! user with that email address - it will fail validation client-side.

• The EmailRule class doesn't accept test@joomla.org.:
  https://github.com/joomla/joomla-cms/blob/staging/libraries/src/Form/Rule/EmailRule.php
  To test, run the following:

$rule = JFormHelper::loadRuleType('email');
$field = new SimpleXMLElement('<field></field>');
var_dump($rule->test($field, 'test@joomla.org')); // this works
var_dump($rule->test($field, 'test@joomla.org.')); // this fails

Seems like an inconsistency in how MailHelper::isEmailAddress() is validating emails.

Issue was related to OpenID:
https://developer.joomla.org/joomlacode-archive/issue-12217.html
http://joomlacode.org/gf/project/joomla/scmsvn/?action=browse&path=%2F&view=rev&revision=11380

Specifically, this part in OpenID plugin:

$response->email = str_replace( array('http://', 'https://'), '', $response->username );
$response->email = str_replace( '/', '-', $response->email );
$response->email .= '@openid.';

Thank you so much for digging this up!
Ok, so since this plugin no longer exists I assume I can create a MR to fix this and see if any test units are failing.

Set to "closed" on behalf of @franz-wohlkoenig by The JTracker Application at issues.joomla.org/joomla-cms/20075

closed as having Pull Request #20080.