So far, pseudonyms and domains can only be used in the context of studies. For study pseudonyms this makes a lot of sense. But for general domains there can be situations where you want to find information about a subject without the context of a specific study. So !2183 (merged) added an exception for that. This is another exception.
Say someone is in charge of blood samples. Some day, one of the samples is stolen. The person in charge wants to inform the subject that their blood has been stolen. With this new functionality, they can find the subject by pseudonym. There previously was no real way to do that (other than using a DB console).
So far we were reluctant to add this feature because pseudonyms from general domains are persistent identifiers that can be used outside of the limited context of a study. However, names are also persistent identifiers, and this feature is still far more restrictive than the way we handle those.