Yeah, and I’ve been annoyed for a long time about the inconsistency because I feel all should be the same. But I looked at the code and the code for artist/create is from 2011. So the only options I see are a) a redirect used to exist that got removed for some reason or b) that link never worked.
I cannot find anything about https://musicbrainz.org/artist/add redirect in the codebase history. If it ever existed, it may have been set up outside the codebase and may have been dropped when MusicBrainz servers moved to new host in 2016 for example, or any earlier hosting change.
I dont know if it worked recently since I havent used it for some time, but it did work a few years ago after I originally created Albunack, which was many years after MusicBrainz NGS