Split Artists should remove the artist being split, but it appears that it can't do that if the artist is involved in a "performed at event" relationship. There is, apparently, no way to provide an artist credit involving several artists when editing the event page, so the Event objects seem to be preventing Split Artists from doing its job.

This is what I presume is happening with the artist https://musicbrainz.org/artist/e53f896b-edb1-4931-9af1-cd86ffc64de9 "Emmylou Harris & Rodney Crowell". A Split Artists was done nearly two months back, but the collaboration artist is still there, and there are no other apparent relationships still tied to the old artist.