It isn't part of the main storyline, but it's presented as a separate spin - off. (metalgearinformer.com)
We most likely will not know before Fire Emblem Warriors has launched, because that shield is seemingly a central part of the main storyline. (perfectly-nintendo.com)
Although these quests are not part of the main storyline, some of them do need to be completed in order to advance the game's story. (thevitalounge.net)