Not sure how widespread this is. After multiple artists are merged, the old artists are not removed from the database.

Examples:

http://musicbrainz.org/edit/31654441 – "Steven Demetre Georgiou" was removed, but "Cat Stevens" was not (and still shows the yellow highlighting).

http://musicbrainz.org/edit/31211453 – Four previous artists are still extant in the database despite the merge completing, also with the yellow highlighting.