The documentation for Picard’s Cover Art Options states:
You must enable “Options ‣ Metadata ‣ Use release relationships” for Picard to be able to download cover art from MusicBrainz cover art relationships.
But what if I don’t want release relationships tags on my files? These tend to change frequently and appear as “files needing fixes” for my library, but I intend to use it in a media player where those won’t be visible (but cover art will!)
I suppose I could, maybe, find the complete list of tags that Release Relationships adds, and blacklist them all (Ignore tags), but this would still allow Cover Art? I don’t know, this seems like a lot of work, and I think it’d be good to support getting Cover Art without requiring Release Relationships enabled.