The second parameter `$plaintext_pass` was removed in [33023] and restored as `$notify` in [33620] with a different behavior. If you have a plugin overriding `wp_new_user_notification()` which hasn't been updated you would get a notification with your username and the password "both".
To prevent this the second parameter is now deprecated and reintroduced as the third parameter.
Adds unit tests.
Merge of [34116] to the 4.3 branch.
Props kraftbj, adamsilverstein, welcher, ocean90.
See #33654.
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