Also mentioned as part of MBS-4937, but that ticket is not very "atomic":
If an artist or any other entity gets merged, the old MBID page is still considered "valid":
https://musicbrainz.org/artist/f21a407e-3af9-4539-ab3d-c92a5230dff6

Instead, it would be better if it would HTTP redirect to the "new" MBID:
https://musicbrainz.org/artist/1155431a-d35e-4863-9ae0-e3c24eb61aa9

This would make it apparent that it now "lives" on the "new" MBID, which would prevent e.g., search engine from thinking the same content on two different pages (and score the page lower as a result).