I can understand using the [unknown] artist in places where you absolutely need to give an artist. But for ARs like here:
What would be the purpose? Signal that there is an orchestra on the recording, although we known nothing about it?
Specifying [unknown] as the second recording artist like here
seems even stranger to me. Maybe someone’s reads the second bullet point of https://musicbrainz.org/doc/Style/Classical/Recording/Artist so this is required? I understood it to say, that only if no performer is known, we use [unknown].