Hi @Orygaw, thanks for the suggestion. Unfortunately at the moment there isn't a way to disable the administrator emails for new user creation or deletion. I'm not aware of any plugin that adds this option.

Would it be useful to have a "send email notifications for user creation and deletion" setting, on by default, which could be turned off?

I agree. An option for those mails might be nice.
If you want to disable creation and deletion mails on your instance you can currently achieve that by creating something like this in config/config.php

<?php

return [
    'observers.global' => \DI\add([
        ['Mail.shouldSend', \DI\value(function (&$shouldSendMail, $mail) {
            if ($mail instanceof \Piwik\Plugins\CoreAdminHome\Emails\UserCreatedEmail ||
                $mail instanceof \Piwik\Plugins\CoreAdminHome\Emails\UserDeletedEmail) {
                $shouldSendMail = false;
            }
        })],
    ]),
];

Would it be useful to have a "send email notifications for user creation and deletion" setting, on by default, which could be turned off?

Although there is no place for mail notification option inside general settings web frontend (CoreAdminHome) maybe in first iteration, a simple config.ini.php parameter is enough.