Drop UserMailRequired constraint
For those that find themselves here trying to make User Emails Optional, use hook_validation_constraint_alter. You're not extending core's, you're creating a new constraint and using it instead of core's. In my case, I replace core's with... an empty function to achieve what I want.

Something like this in .module

function optional_account_email_validation_constraint_alter(array &amp;$definitions) {
  $definitions['UserMailRequired']['class'] = '\Drupal\my_module_name\Constraint\OverrideUserMailRequired';
}


Then a copy of the two UserMailRequiredValidator.php and UserMailRequired.php files changed to your liking.

The module administerusersbyrole has a working example of this. 

I'll see if I can get around to putting my custom module onto drupal.org in the coming months.