From PTR patch notes: Collecting all 21 Ethereals during Season 24 will reward players with the Feat of Strength, Ethereal Recollection. Players who accomplish this will have all Ethereal transmogrify options available for future seasonal and non-seasonal play.
From what I understand, achieving the Feat of Strength grants the transmog unlock for future seasonal + non-seasonal play. Does that mean I do not need to farm on both hardcore and non-hardcore modes and getting all 21 on a single mode should suffice?